题目内容：

    In space, there is no up or down. Without the gravity that we experience on Earth, as-tronauts and everything else on a spaceship float freely.

Humans can experience the sense of zero gravity without going into space. How？A zero-gravity flight. When an airplane flies in a lot of parabolas（抛物线）, a man-made zero-gravity environment is set up. Everything in the plane floats, even passengers.

Gravity has an effect on everything from the human body and mind to the behavior of plants and the way machines run. Space travel is expensive and dangerous. But zero-gravity flights let scientists conduct experiments in an environment that mimics（模拟） the conditions of space. On each of its research trip, Zero-G flies about 30 parabolas, each of which creates a period of weightlessness that lasts about 30 seconds. Nikolaus Kuhn of the University of Ba-sel, in Switzerland, flew with Zero-G. He was conducting an experiment about soil and the flow of water on Mars. The Red Planet has about one-third of the gravity that Earth has. This means water flows more slowly on Mars. By making the parabolas less steep, zero-gravity flights can make the feeling of Martian gravity. Flying with Zero-G has been necessary for Kuhn’s research. “It is the only chance, other than going to Mars, to mimic movement of water as it would be on Mars,” he told TFK.

For years, NASA used zero-gravity flights to prepare and train astronauts. “I have not been to space,” Peters of Zero-G says, “But I have been told by astronauts that zero-gravity feels exactly the same.” As humans continue to explore the universe beyond Planet Earth, zero-gravity flights will remain an important and fun tool. “It never gets old,” Peters says “I would do it every weekend for the rest of my life if I could.”

1.Why do scientists set up man-made zero-gravity environment？

A.To study the water on the earth.

B.To offer weightlessness as a business.

C.To take scientists to any other planets.

D.To mimic the condition of space for experiments.

2.How much time of weightlessness can a Zero-G light offer？

A.About 30 seconds. B.About 90 seconds.

C.About 15 minutes. D.About 30 minutes.

3.What feature do the parabolas in Zero-G flights have？

A.The gentler parabolas are, the more satisfied the researchers are.

B.The steeper parabolas are, the less weight the passengers feel.

C.The steeper parabolas are, the more successful the experiments are.

D.The gentler parabolas are, the more quickly water on Mars flows.

4.What can be inferred about the Zero-G flight from the text？

A.It is useless for training astronauts.

B.It has no side effect on human body.

C.It creates a substitutable environment of zero gravity in space.

D.It is cheaper but more dangerous than space travel.