Question 21 is based on the passage you have just heard.

    (19) One thing about the moon many people don’t know is that it has a lot of garbage on its surface, leftover from human space exploration. But how much garbage exactly have humans left on the moon? It’s hard to be accurate, but the trash likely weighs more than 181,000 kilograms on Earth. Much of it was left by American astronauts, who landed on the lunar surface between 1969 and 1972 during NASA’s Apollo Missions. The other rubbish comes from missions that did not have human crews. These missions were conducted by various space exploring agencies, including those from the US, Russia, Japan, India and Europe. Many of the older pieces are equipment sent to learn about the moon. The equipment stayed there after its mission ended. The moon is also home to lunar orbiters that mapped the moon before they crashed into its surface, adding to the garbage heap. The objects left by the Apollo astronauts included the equipment that was no longer needed. (20) Bringing back unneeded equipment would have used up precious resources such as fuel. But, as the saying goes, “one person’s trash is another’s treasure”, (21) researchers can study the garbage left on the moon to see how its materials weathered the radiation and vacuum of space overtime. Moreover, some of the objects on the moon are still being used, including a laser-range reflector left by the Apollo 11 crew.

21. What can researchers do with the garbage on the moon?

解析：录音最后指出，研究人员可以研究留在月球上的垃圾，看看其材料是如何经受住长时间太空辐射和真空的。C选项是对原文信息的同义转述，故为正确答案。