问题：

[单选,材料题] 请在第__64__处填上正确答案。

根据材料，完成下面的题目。

UFOS

       Sixty years ago, a. man named Kenneth Arnold saw something that we can still (51) today -- something that changed popular culture for ever.
Flying his plane over mountains in the U. S. state of Washington, he saw a,line of strange objects, either crescent-shaped (月牙形的) or disc-like, flying (52) the motion of a saucer (碟) skimming (飞速掠过) on water.
       The media soon picked up on the story -- the Flying Saucers were here] Was the earth being (53) by creatures from another planet? Soon, So many sightings were made that the U. S. military began to (54) It called these strange Objects UFOs – Unidentified Flying Objects, and that is how they are (55) today.
       Military investigations found no evidence of visitors from outer space. But that did not stop the true (56) The military were (57) up, they said. Or maybe it was because the travelers from space were of such superior intelligence (58) they could hide from military analysts (分析家).
People have always seen strange lights in the sky. In the past these were explained in (59) ways. In a world where religion was less influential and science fiction was popular, signs from God were replaced by visitors from other (60).
       The date of the first UFO sightings was also significant. In 1947, World War II had just ended and the (61) War was just .beginning. Humanity seemed locked in endless conflicts. Like generations before them, people looked (62) the skies for help. But instead of seeking God, they looked for help from super-intelligent aliens (外星人) with (63) technology. Belief in UFOs became the first religion of science.
However, even people who believe in UFOs are not quite sure why they visit the earth.The universe is a big place and it is (64) to assume that there is life somewhere out there. It is possible that aliens have worked out how to travel through space. Yet some people report that they have been taken by aliens and have had experiments (65) on them. Why would anyone travel across half the universe to conduct medical experiments on people living in small towns in the United States?
请在第__51__处填上正确答案。

A . unthinkable
B . Impossible
C . reasonable
D . unimaginable