Read the following article and select the best answer. Article: It just may be the world's most costly vacuum cleaner ( ). The price of CleanSpace One is eleven million dollars. But yet, it has a big job to do. Recently, researchers in Switzerland said they planned to build this new cleaning machine. CleanSpace One will not be for use in homes or businesses. It will be shot into space to help remove thousands of pieces of space junk floating around up there. Last year, we reported on the problem of space junk. Over time, many unused spacecraft have hit each other far above the Earth. Big pieces break into thousands of small pieces. Sometimes they fall back into the atmosphere and burn up. But when they do not, it creates big problems. Scientists fear that if something is not done to remove these objects, it may soon become too dangerous to send people and machines into space. There is a large chance that they might hit some of this junk. So that is why the Swiss researchers are developing the new machine. CleanSpace One is not really a vacuum cleaner. It will not be used to take away space junk. But scientists plan to move it close to an old satellite that is no longer being used. Then a claw-like instrument will catch the satellite, and force it back through the atmosphere. There, the satellite will be destroyed by the heat of friction with the air. Researchers say that all they need to do is slow down the speed of some of these unwanted objects. Once they begin to move more slowly, they will fall back to the Earth. There is little chance that they will fall through the atmosphere and harm the people or things below. Question: Unwanted objects will fall back to the Earth when _ . - they are made to move slowly - they meet other objects in space - they are not used for years - they are near the atmosphere
A:
they are made to move slowly