Question 20 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.

20. Why has a lot of equipment being left on the moon?

解析：录音后半部分指出，带回不需要的设备会消耗宝贵的资源，例如燃料。也就是说，带回它们的成本很高，故A选项为正确答案。C选项为强干扰项，录音中提到这些设备是无用的，但并未说明在地球上没有用处，故C选项错误。