Evidence has been put forward showing that astronauts exposed to long periods of weightlessness _ quite severely.
问题描述：
Evidence has been put forward showing that astronauts exposed to long periods of weightlessness _ quite severely.
A.have affected
B.had affected
C.have been affected
D.had been affected
答案是选c,为什那么不用用had而用have
答
.显然是have,它在描述一种事实,而不是完成的时态,你可能看到了exposed这个词,但是这不是过去式,你去查下字典就知道了,又不是加了ed就代表过去式