请教PREP－12

12. The success of the program to eradicate smallpox has stimulated experts to
pursue what they had not previously considered possible -- better control, if not eradication, of the other infections such as measles and yaws.

A. what they had not previously considered possible -- better control, if not eradication, of the other infections such as

B. what they had not previously considered a possibility -- better control, if not
eradication, of such infections like

C. something they had not previously considered possible -- better control, if not
eradication, of such infections as

D. something not considered a previous possibility -- better control and perhaps
eradication, of other infections such as

E. the possibility of what they had not previously considered possible -- better
control and possibly eradication of infections like

答案好像时C吧，对大案没有什么问题

不明白这里为什么要用过去完成时？

是C

时态是我的弱项，不敢乱说。

不知道Zeldaw兄你还记不记得我问过的一道题，当时很多人说现在完成时表示的过去的时间点。
http://forum.chasedream.com/dispbbs.asp?BoardID=23&replyID=2225913&id=237175&skin=0

如果这个说法是对的话，那么这道题应该就是印证了这个说法：在 has stimulated 这个过去的时间之前认为不可能，所以用过去完成时。