① Man has known more and more about space today. Space is not only wonderful but also dangerous. While working in space, astronauts are facing danger.

② Scientists have found out that radiation (辐射) is the greatest danger to astronauts in space. When astronauts are working in space, they are in danger of radiation from the sun and other stars, which is bad for their health. The damage of radiation is difficult to discover until their children or even grandchildren are born. Some special medicine may work a little, but there isn’t really helpful medicine.

③ Space rubbish is also a great danger to astronauts. There are 9000 man-made (人造) things flying in space. About 30% of these are man-made satellites (卫星), 10% are spaceships, and others are space rubbish. Even a small piece of these knocked against a spaceship window and did some damage. Scientists are watching and telling any possible danger all the time. They are working on helpful ways to do with space rubbish.

④ Although space is really dangerous, everyone on the earth is interested in it. In the near future, it may become usual for people to spend a few days in a space hotel. We may even choose to leave the earth and live in space for a few years. So we should care for space and do something to help improve the space environment.

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B．

C．

A．

B．

C．

Three Journeys into Space

Saturn (土星) has beautiful rings. Of planets in the solar system, Saturn’s rings are the biggest and brightest. According to scientists, however, these rings will one day disappear completely.

How did Saturn get its rings in the first place? Scientists have no sure answer. But there’s a popular theory (理论): Some comets (彗星) or small planets ran into Saturn’s moons (卫星). Saturn had at least 60 moons. And this left behind a large number of ice and rock pieces. These pieces were different in size. Some were as small as grains of sand, Others were as large as houses.

These small pieces began to travel around Saturn. Saturn’s gravity (引力) pulled them towards it. At the same time, the speed of their orbit (轨道) tried to throw them into space. Over time, these pieces of ice and rock formed rings around Saturn.

A new study, however, suggests that Saturn’s gravity is winning. About ten tons of “ring matter” is falling into Saturn every second. If this continues, the planet’s ring system will disappear completely in 100 million to 300 million years time. According to scientists, Saturn’s rings are between 10 million and 100 million years old. “We’re lucky to be able to see Saturn’s huge ring system,” one of the scientists said, “Some of the planets in the solar system only have very thin rings today. Perhaps we just missed out on seeing their huge ring systems!”