[判断题]预防式离机选用正常的登机口，不使用滑梯。
A.正确
B.错误

Black holes. They act like huge drains in the universe. Frightening Fascinating Or just fantasy The very concept of black holes confuses our common sense notions of matter, time, and space. The theory that black holes can, and probably do, exist in space doesn’ t only concern scientists and astronomers.
61. {{U}}For if the theory is valid, we must question all our basic physical laws and, indeed, our "normal" experience of the physical world around us.{{/U}}
What is a black hole Well, it’ s difficult to answer this question, since the terms we would normally use to describe a scientific phenomenon are inadequate here. 62. {{U}}Astronomers and scientists think that a black hole is a region of space (not a thing) into which matter has fallen and from which nothing can escape--not even light.{{/U}} So we can’ t see a black hole. A black hole exerts a strong gravitational pull and yet it has no matter. It is only space--or so we think.