[C] They are used for rescue work.(A)

[D] They are now used exclusively for commercial projects.

15. Under what conditions are helicopters found to be absolutely essential?

[A] For overseas passenger transportation.

[B] For extremely high altitude flights.

[C] For high-speed transportation.(D)

[D] For urgent mission to places inaccessible to other kinds of craft.

Unit four

Test 1

A scientist once said: “I have concluded that the earth is being visited by intelligently controlled vehicles from outer space.”

If we take this as a reasonable explanation for UFOs (unidentified flying objects), questions immediately come up.

“Why don’t they get in touch with us, then? Why don’t they land right on the White House lawn and declare themselves?” people asked.

In reply, scientists say that, while this may be what we want, it may not necessarily be what they want.

“The most likely explanation, it seems to me,” said Dr. Mead, “is that they are simply watching what we are up to -- that responsible society outside our solar system is keeping an eye on us to see that we don’t set in motion a chain reaction that might have unexpected effects for outside our solar system.”

Opinions from other scientists might go like this: “Why should they want to get in touch with us? We may feel we’re more important than we really are! They may want to observe us only and not interfere with the development of our civilization. They may not care if we see them but they also may not care to say ‘hello’.”

Some scientists have also suggested that Earth is a kind of zoo or wildlife reserve. Just as we set aside wilderness areas and wildlife reserves to allow animals and growing things to develop naturally while we observe them, so perhaps Earth was set aside ages ago for the same purpose.

Are we being observed by intelligent beings from other civilizations in the universe? Are they watching our progress in space travel? Do we live in a gigantic “zoo” observed by our “keepers,” but having no communication with them?

Never before in our history have we had to confront ideas like these. The simple fact is that we, who have always regarded ourselves as supreme in the universe, may not be so. Now we have to recognize that, among the stars in the heavens, there may very well be worlds inhabited by beings who are to us as we are to ants.

16. People who ask the question “Why don’t they get in touch with us... and declare themselves?” think that ________.