Human beings have a natural desire to explore the unknown. People with a strong interest in space like to say it is the last place left to explore. But scientists are warning that space exploration for long periods is not going to be easy. They say the human body is not designed to stay in space over an extended period.
The New York Times recently published a report about scientists who are preparing astronauts for a trip to the Moon. The newspaper said the scientists want to make sure that their crews return home in good health. But there are many problems to be solved before people are ready for long trips to the Moon, an asteroid or even the planet Mars.
Humans developed on a planet with a surface that is more than 70 percent water. Our bodies are also about 70 percent water. When there is no gravity that water moves up toward the head, raising pressure in the skull. Arms and legs grow weaker at what is called zero gravity because they no longer need to push against the force of gravity.
Five years ago, astronauts who spent weeks in space reported a change in their eyesight. These astronauts were members of the crew on the International Space Station. Research showed a change in the shape of their eyes. Normally-round eyeballs had become flat during time in space. The research also showed that the right eye was affected more than the left, and that men were more affected than women. Scientists could not find an explanation for the differences.
Bone loss was one of the problems first reported by astronauts returning to Earth after longer stays in space. So scientists designed exercise machines to use on the space station. Tests showed that the exercise equipment helped space travelers keep their bones almost as strong as when they left Earth.
There are other health issues for astronauts who spend a long time in space. They may have problems eating and sleeping. But the biggest health issue is exposure to radiation. On Earth, human beings are protected by the atmosphere and the planet's magnetic field. In outer space, there is no such protection.
What is the main idea of the passage？

A．Long periods in space will affect the health of astronauts.

B．Scientists are trying to find out how to protect astronauts.

C．Space is dangerous to human beings.

D．People shouldn’t explore space.

What will happen to human bodies in space？

A．Arms and legs grow stronger at what is called zero gravity

B．Blood moves up toward the head.

C．Pressure in the skull will be raised.

D．All of the above.

The following are problems mentioned in the passage Except________.

A．bone loss

B．loss of water

C．eyesight

D．exposure to radiation

Why did scientists design exercise machines to use on the space station?

A．To make sure that their crews return home in good health.

B．To help astronauts keep their bones almost as strong as when they left Earth.

C．To help astronauts get ready for long trips to the Moon, an asteroid or even the planet Mars.

D．To let astronauts adapt to the environment of space.

What is true according to the passage?

A．The biggest health issue is exposure to Sunshine.

B．Research showed a change in the size of their eyes.

C．The right eye was affected less than the left.

D．Men were more affected than women.