There is a unified and central location for currency trading.

A、vary

B、variable

C、variation

D、variant

A、start

B、startdown

C、end

D、start-up

The reason that these that these two wonders of nature are so difficult for many adults to explain to children is that they are not very well understood by adults themselves. For example, did you know that the lighting we see flashing(闪烁) down to the earth from a cloud is actually flashing up to a cloud from the earth？ Our eyes cheat us into thinking we see a downward motion when it’s actually the other way round. But then, if we believe only what we think we see, we’d still insist that the sun rises in the morning and sets at night.

Most lightning flashes take place inside a cloud, and only a relative few can be seen jumping between two clouds or between the earth and a cloud. But, with about 2,000 thunderstorms taking place above the earth every minute of the day and night, there’s enough activity to produce about 100 lightning strikes on the earth every second.

Parents can use thunder and lightning to help their children learn more about the world around them. When children understand that the light of the lightning flashes reaches their eyes almost at the same moment, but the sound of the thunder takes about 5 seconds to travel just one mile, they can begin to time the interval between the flash and the crash to learn how close they are to the actual spark.

26. According to the first paragraph, in the Central Valley area, rains come _____.

A. when it is April

B. when the field is dry

C. when people all hope for it

D. usually without thunder and lighting

27. We tend to think that lighting moves downward because_____.

A. our eyes play a joke on us

B. we take it to be true scientifically

C. we cannot see it clearly most of the time

D. it always runs down from a cloud to the earth

28. Which of the following is NOT true about lighting according to the passage？

A. Most lightning flashes take place inside a cloud.

B. About 2,000 thunderstorms occur above the earth every minute.

C. People can seldom see lightning flashes running between two clouds or between the earth and a cloud.

D. The thunderstorms happening above the earth can provide enough power to produce about 100 lightning strikes every second.

29. What does the word “ spark” in the last paragraph probably refer to ？

A. Cloud

B. Rain

C. Thunderstorm

D. Lightning flash

30. What does the passage imply？

A. Seeing is believing.

B. We should never trust what we see or hear.

C. People may easily make mistakes in their understanding of nature.

D. Children sometimes know more about nature than their parents do.

B．simplified?management?of?central?or?remotely?located?access?points

C．extension?features?to?wired?network?services，?such?as?denial?of?service?prevention?and?ACLs

D．threat?defense?with?a?robust?wired?IDS