阅读下列短文，从各题所给的A、B、C和D项中选出最佳选项。
Many people know that rubbish is a big problem on planet Earth. What many people don't know is that junk(垃圾)has become a problem in outer space too.
According to BBC News, there are more than 22, 000 pieces of space junk floating around the earth. Andtheseare just the things that we can see from the surface of the earth by telescopes (望远镜). There are also millions of smaller pieces of junk that we can't see.
Objects, like bits of old space rockets or satellites, move around the planet at very high speeds fast that even a very small piece can break important satellites or become dangerous to astronauts. If the tiniest piece of junk crashed into a spaceship, it could damage the vehicle.
To make things worse, when two objects in space crash, they break into many smaller pieces. For example, when a U.S. satellite hit an old Russian rocket in 2009, it broke into more than 2,000 pieces, increasing the amount of space junk.
To reduce additional space junk, countries have agreed that all new space tools can only stay in space for 25 years at most. Each tool must be built to fall safely into the earth's atmosphere after that time. In the upper parts of the atmosphere, it will burn up.
Many scientists also suggesting different ways to clean up space junk. In England scientists are testing a metal net that can be fired into space junk. The net catches the junk and then pulls it into the earth's atmosphere to burn up. The Germans are building robots that can collect pieces of space junk and bring them back to Earth to be safely destroyed.
The problem is becoming more challenging because we're sending more objects into space to help people use their mobile phones and computers,” says Marco Castronuovo, an Italian space Researcher.
“The time to act is now. The longer we leave the problem, the bigger it will become,” he says.
