题目内容
（请给出正确答案）
Scientists counted the number of stars________.
A．to illustrate the impact of light pollution
B．to compare air quality in different areas
C．to see how the sky has changed with time
D．to arouse public interest in space
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更多“Scientists counted the number of stars________.A．to illustrate the impact of light polluti”相关的问题
Scientists counted the number of stars in order to______.
A．illustrate the impact of light pollution
B．compare air quality in different areas
C．see how the sky has changed with time
D．arouse public interest in space
Radiocarbon Dating
Nowadays scientists can answer many questions about the past through a technique called radiocarbon(放射性碳), or carbon-14, dating. One key to understanding how and why something happened is to discover when it happened.
Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at the University of Chicago. An atom of Ordinary carbon, called carbon-12, has six protons(质子) and six neutrons(中子) in its nucleus(原子核). Carbon-14, or C-14, is a radioactive, unstable form. of carbon that has two extra neutrons. It returns to a more stable form. of carbon through a process called decay(衰减). This process involves the loss of the extra neutrons and energy from the nucleus.
In Libby's radiocarbon dating technique, the weak radioactive emissions(放射) from this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion of C-14 atoms in the sample being dated.
Carbon-14 is produced in the Earth's atmosphere when nitrogen(氮)-14, or N-14, interacts with cosmic rays(宇宙射线). Scientists believe since the Earth was formed, the amount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now ratio of C-14 to other carbon atoms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years.
All life on Earth is made of organic molecules(分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14 atoms to other carbon atoms in their tissues(组织). Once an organism(有机体) dies it stops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is.
Nowadays many scientists depend on radiocarbon for dating age-old objects.
A．Right
B．Wrong
C．Not mentioned
Radiocarbon Dating
Nowadays scientists can answer many questions about the past through a technique called radiocarbon(放射性碳), or carbon-14, dating. One key to understanding how and why something happened is to discover when it happened.
Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at the University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(质子) and six neutrons(中子) in its nucleus. Carbon-14, or C-14, is a radioactive, unstable form. of carbon that has two extra neutrons(原子核). It returns to a more stable form. of carbon through a process called decay(衰减). This process involves the loss of the extra neutrons and energy from the nucleus.
In Libby's radiocarbon dating technique, the weak radioactive emissions(放射) from this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion of C-14 atoms in the sample being dated.
Carbon-14 is produced in the Earth's atmosphere when nitrogen(氮)-14, or N-14, interacts with cosmic rays(宇宙射线). Scientists believe since the Earth was formed, the amount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now ratio of C-14 to other carbon atoms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years.
All life on Earth is made of organic molecules(分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14 atoms to other carbon atoms in their tissues(组织). Once an organism(有机体) dies it stops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is.
Nowadays many scientists depend on radiocarbon for dating age-old objects.
A．Right
B．Wrong
C．Not mentioned
A、A. His swimming skill is too poor to survive.
B、B. His desired job doesn't pay much.
C、C. He didn't get the job he counted on.
D、D. He doesn't have enough money now.
There are only ten apples left in the baskets, ______ the spoilt ones.
A．not counting
B．not to count
C．don't count
D．having not counted
63 There are only ten apples left in the baskets, __ the spoilt ones.
A not counting B not to count C don’t count D having not counted
听材料，回答下列问题：
A.Because it was discovered after Mount Everest．
B.Because most of the mountain is underwater．
C.Because a volcano can’t be counted as a real mountain．
D.Because it locates on the island in the ocean．
A．He shouldn't have counted on the chances of not being exposed.
B．It is hard to predict whether the Bush Administration would be more "pragmatic" and easier to work with or not.
C．Moscow had to back down a bit, stressing its willingness to talk about a missile shield.
D．Both A and Ｂ.
A．He shouldn't have counted on the chances of not being exposed.
B．It is hard to predict whether the Bush Administration would be more "pragmatic" and easier to work with or not.
C．Moscow had to back down a bit, stressing its willingness to talk about a missile shield.
D．Both A and B
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