我一直以为定语从句，宾语从句的引导词，在GMAT中是不可以省略的，但是PREP里有这样的题目

17.   (25335-!-item-!-188;#058&002335)

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--better control and possibly eradication of infections like

C就是被我直接排除的，理由就是sth后面没有定语从句先行词，不过我做了这么多题，这好像是第一道省略引导词的

所以想问问，是否省略？如果是小概率事件，我索性忘记这题