【答案解析】[听力原文]
Unidentified Flying Objects
There are many explanations for why UFOs visit the earth. /The most popular one is that they maybe visitors from other planets. /To fly such aircraft, their builders must develop different forms of aviation, /because they seem to fly much faster than normal aircraft. /The UFOs, it is believed, must contain scientists/from other planets who are studying life on earth. /It is even believed that several such aircraft may have landed on earth/and the space visitors may be living amongst us./
But there are also less fantastic explanations available. /Although some sightings of UFOs are difficult to explain, /most can be explained quite easily. /In many cases the observers might have made a mistake. /They might have seen a weather balloon or an aircraft. /Or the light they saw in the sky might have been light from the ground, reflected on to the clouds. /However, the exact cause of many sightings still remained a mystery./