As is known to all,the threats to the health of astronauts in space are very difficule from____on ea

题目

As is known to all,the threats to the health of astronauts in space are very difficule from____on earth Ait B that Cones Dthose

答案

答案应该选D,这里的those指的是同前面的astronauts同等级的,在地球上自然就是指人类了,而且应该用复数形式,astronauts同样也是复数形式!