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When must the sonar devices be placed？
Before the ______.
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更多“When must the sonar devices be placed？Before the ______.”相关的问题
The sonar devices must be placed ______.
A．from time to time
B．after the undersea vehicle dives
C．at the time when the undersea vehicle is diving
D．before the undersea vehicle dives
century by the Italian naturalist Lazzaro Spallanzani.He caught some bats in a bell tower，blinded them，and released them outdoors.Four of these blind bats were recaptured after they had found their way back to the bell tower，and on examining heir stomachs‘ contents，Spallanzani found that
they had been able to capture and gorge themselves with flying insects.We know from experiments that bats easily find insects in the dark of night ， even when the insects emit no sound that can be heard by human ears.A bat will catch hundreds of soft-bodied，silent-flying moths or gnats in a single hour.It will even detect and chase pebbles or cotton spitballs tossed into the air.
16.According to the author，the sonar system of bats is an example of the idea that.()
A.this is the age of technological triumphs
B.modern machines are inefficient
C.living mechanisms are often more efficient than man-made
machines
D.artificial imitations are always less efficient than living
mechanisms
17.The author suggests that the sonar system of bats ().
A.was at the height of its perfection 50 million years ago
B.is better than man-made sonar because it has had 50 million years to be refined
C.would have been discovered by man many years ago
D.is the same as it was 50 million years ago
18.The fact that“blind bats”will detect and chase cotton spitballs as well as insects is remarkable because.()
A.bats do not eat spitballs
B.cotton is harder to track
C.Spitballs make no sounds audible to human ears
D.there is purpose in the flight of insects
19.This passage was written to illustrate().
A.the deficiencies of man-made sonar
B.the dependence of man upon animals
C.that we are living in a machine age
D.that the sonar system of bats is remarkable
20.Which of the following is the main point of the passage？()
A.A bat will catch hundreds of gnats in a single hour
B.Here is a perfection in nature which sometimes cannot be matched by man‘s creative efforts
C.The phrase“blind as bat”is valid
D.director of NIH learned of the abuse
A．the whales may create detectable "sound shadows"
B．the new sonar equipment is more powerful
C．the new sonar technology can see through sea water
D．the existence of whales may reinforce reflection signals
【填空题】Task 1 Fill in the blanks with the right words or phrases. Change the form where necessary. start with pay attention countdown although as far ahead as possible prefer pay off incentive whereas related to 1. Gorge kept his coat on, _______ it was warm in the room. 2. When you set the date, please inform us _________________________. 3. Radar employs radio waves ________ sonar uses sound waves. 4. Which one do you _____, a glass of sherry or a cocktail？ 5. We should __________ to the issue of environmental protection. 6. The _______________ was well under way. 7. Let’s ________ our party _________ a light music. 8. Your efforts will ___________ in the long run. 9. The management is taking ___________ measures to increase the company’s productivity. 10. There are many job opportunities ____________ my major.
Text 3
When prehistoric man arrived in new parts of the world, something strange happened to the large animals. They suddenly became extinct. Smaller species survived. The large, slow-growing animals were easy game, and were quickly hunted to extinction. Now something similar could be happening in the oceans.
That the seas are being overfished has been known for years. What researchers such as Ransom Myers and Boris Worm have shown is just how fast things are changing. They have looked at half a century of data from fisheries around the world. Their methods do not attempt to estimate the actual biomass (the amount of living biological matter) of fish species in particular parts of the ocean, but rather changes in that biomass over time. According to their latest paper published in Nature, the biomass of large predators (animals that kill and eat other animals) in a new fishery is reduced on average by 80% within 15 years of the start of exploitation. In some long-fished areas, it has halved again since then.
Dr. Worm acknowledges that the figures are conservative. One reason for this is that fishing technology has improved. Today’s vessels can find their prey using satellites and sonar, which were not available 50 years ago. That means a higher proportion of what is in the sea is being caught, so the real difference between present and past is likely to be worse than the one recorded by changes in catch sizes. In the early days, too, longlines would have been more saturated with fish. Some individuals would therefore not have been caught, since no baited hooks would have been available to trap them, leading to an underestimate of fish stocks in the past. Furthermore, in the early days of longline fishing, a lot of fish were lost to sharks after they had been hooked. That is no longer a problem, because there are fewer sharks around now.
Dr. Myers and Dr. Worm argue that their work gives a correct baseline, which future management efforts must take into account. They believe the data support an idea current among marine biologists, that of the “shifting baseline.” The notion is that people have failed to detect the massive changes which have happened in the ocean because they have been looking back only a relatively short time into the past. That matters because theory suggests that the maximum sustainable yield that can be cropped from a fishery comes when the biomass of a target species is about 50% of its original levels. Most fisheries are well below that, which is a bad way to do business.
31. The extinction of large prehistoric animals is noted to suggest that ________.
[A] large animal were vulnerable to the changing environment
[B] small species survived as large animals disappeared
[C] large sea animals may face the same threat today
[D] slow-growing fish outlive fast-growing ones
That the seas are being overfished has been known for years. What researchers such as Ransom Myers and Boris Worm have shown is just how fast things are changing. They have looked at half a century of data from fisheries around the world. Their methods do not attempt to estimate the actual biomass (the amount of living biological matter) of fish species in particular parts of the ocean, but rather changes in that biomass over time. According to their latest paper published in Nature, the biomass of large predators (animals that kill and eat other animals) in a new fishery is reduced on average by 80% within 15 years of the start of exploitation. In some long-fished areas, it has halved again since then.
Dr. Worm acknowledges that these figures are conservative. One mason for this is that fishing technology has improved. Today's vessels can find their prey using satellites and sonar, which were not available 50 years ago. That means a higher proportion of what is in the sea is being caught, so the real difference between present and past is likely to be worse than the one recorded by changes in catch sizes. In the early days, too, longlines would have been more saturated with fish. Some individuals would therefore not have been caught, since no baited hooks would have been available to trap them, leading to an underestimate of fish stocks in the past. Furthermore, in the early days of longline fishing, a lot of fish were lost to sharks after they had been hooked. That is no longer a problem, because there are fewer sharks around now.
Dr. Myers and Dr. Worm argue that their work gives a correct baseline, which future management efforts must take into account. They believe the data support an idea current among marine biologists, that of the "shifting baseline". The notion is that people have failed to detect the massive changes which have happened in the ocean because they have been looking back only a relatively short time into the past. That matters because theory suggests that the maximum sustainable yield that can be cropped from a fishery comes when the biomass of a target species is about 50% of its original levels. Most fisheries are well below that, which is a bad way to do business.
The extinction of large prehistoric animals is noted to suggest that ______.
A．large animals were vulnerable to the changing environment
B．small species survived as large animals disappeared
C．large sea animals may face the same threat today
D．slow-growing fish outlive fast-growing ones
Manned submersibles (潜水器), like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must simply be sealed against the vacuum of space, a submersible must be able to bear extreme pressure if it is not to break up in deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other hand, men paved the way, only recently have unmanned remote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to these underwater. A vacuum am ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles—particularly ROVs— operate at the end of long ropes.
For a similar reason, knowing where you are undersea is much more difficult than in space. A spaceship's position can be located by following its radio signal, or by using telescopes and radar. For an undersea vehicle, however, a special network of sonar (声纳) must be laid out in advance on the ocean' floor in the area of a dive to locate the vehicle's position.
Though undersea exploration is more challenging than outer space in a number of respects, it has a distinct advantage: Going to the ocean depths doesn't require the power necessary to escape Earth's gravity. Thus, it remains far less expensive.
People did not begin to use unmanned vehicles in undersea exploration until recently because of______.
A．the communication problem
B．the ocean depths
C．the movement of waves
D．the problem of vacuum
Ken Norris, natural history professor, believes that rather than use their teeth to attack their victims, toothed whales stun their prey with intense bursts of sound. He points out that all 68 species of odontocetes (a sub-order that includes dolphins) use sound to locate their victims, and suggests this ability may have evolved to the point where it could be used as a weapon.
Millions of years ago, the ancestors of whales had a narrow tapered jaw, streamlined for speed and bristling with long rows of sharp teeth. Today odontocetes are not adapted to surprising their prey or trapping it quickly. Most are more bulky than their forebearers; their teeth are short and unevenly spaced when they exist at all. Norris suggests that because of their sonar ability, whales' teeth have become vestigial, like the human appendix.
It is difficult to test the big bang theory because in captivity, cetaceans tend to moderate their sonar so as not to deafen other creatures in their small, highly reflective tanks. Dolphins are known to politely turn off their echolocation systems when passing one another. And in the wild it's hard to tell whether a sound came from a nearby animal or form. another very far away.
But Norris points to the male narwhal as tacit proof of his theory. This whale has no teeth, and the single spiral tusk that extends about eight feet in front of him would seem to be more of an impediment than an aid in catching shrimp, his preferred feed. "You look at these animals and you wonder what's going on."
The underlined word enigma means ______.
A．debate
B．fact
C．mystery
D．proof
Manned submersibles, like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must simply be sealed against the vacuum of space, a submersible must be able to bear extreme pressure if it is not to break up in the deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other hand, men paved the way, and only recently have unmanned re- mote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to those underwater. A vacuum is an ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles-particularly ROVs-operate at the end of long ropes.
For a similar reason, knowing where you are undersea is much more difficult than in space. A spaceship's position can be located by following its radio signal, or by using telescopes and radar. For an undersea vehicle, however, a special network of sonar devices must be laid out in advance on the ocean floor in the area of a dive to locate the vehicle's position.
Though undersea exploration is more challenging than outer space in a number of aspects, it has a distinct advantage, going to the ocean depths doesn't require the power necessary to escape Earth's gravity. Thus, it remains far less expensive.
People did not begin to use unmanned vehicles in undersea exploration until recently because of______.
A．the communication problem
B．the movement of waves
C．the ocean depths
D．the problem of vacuum
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