Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34
Scientists estimate that about 35,000 other objects, too small to detect with radar but detectable with powerful Earth-based telescopes, are also circling the Earth at an altitude of 200 to 700 miles. This debris poses little danger to us on the Earth, but since it is traveling at average relative speeds of six miles per second, it can severely damage expensive equipment in a collision. This threat was dramatized by a cavity one-eighth of an inch in diameter created in a window of a United States space shuttle in 1983. The pit was determined to have been caused by a collision with a speck of paint traveling at a speed of about two to four miles per second. The window had to be replaced.
As more and more nations put satellites into space, the risk of collision can only increase. Measures are already being taken to control the growth of orbital debris. The United States has always required its astronauts to bag their wastes and return them to Earth. The United States Air Force has agreed to conduct low-altitude rather than high-altitude tests of objects it puts into space so debris from tests will reenter the Earth's atmosphere and burn up. Extra shielding will also reduce the risk of damage. For example, 2,000 pounds of additional shielding is being considered for each of six space-station crew modules. Further, the European Space Agency, an international consortium is also looking into preventive measures
The word “them" in line 11 refers to which of the following?
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