求助一道题目

wangnancy2003

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic peoples has been established by carbon dating, but what is much more difficult to determine are the use to which primitive peoples put the caves, the reason for their decoration, and the meaning of the magnificently depicted animals.

A.    has been established by carbon dating, but what ismuch more difficult to determine are

B.     have been established by carbon dating, but what ismuch more difficult to determine are

C.    have been established by carbon dating, but thatwhich is much more difficult to determine is

D.     has been establishedby carbon dating, but what ismuch more difficult to determine is   (D)

E.     are established by carbon dating, but that which ismuch more difficult to determine is

RON说what is much more difficult to determine不是主语， the reason for their decoration, and the meaning of the magnificently depicted animals才是，我怎么不明白呐？不是主语从句吗？

wangnancy2003

怎么没人理我呐？

liuce1211

主语肯定是what is much more difficult to determine啊。。AD选的话，关键就是what引导的主语从句谓语动词单复数问题吧。 what is much more difficult to determine主语，谓语动词肯定是is啊。。还有，答案是啥？

wangnancy2003

答案是D