211. The period when the great painted caves at LACE>LascauxLACE> and LACE>AltamiraLACE> were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals. (A) has been established by carbon-14 dating, but what is much more difficult to determine are (B) has been established by carbon-14 dating, but what is much more difficult to determine is (C) have been established by carbon-14 dating, but what is much more difficult to determine is (D) have been established by carbon-14 dating, but what is much more difficult to determine are (E) are established by carbon-14 dating, but that which is much more difficult to determine is The best answer is B. But I donot agree. I think that "what is much more difficult to determine is "should be "what are much more difficult to determine are", since three things-- the reason, the use and the meaning are difficult to determine. So, no answer is correct. Who would like to explain more to me? Thank you for your help.

名词性从句做主语，谓语通常为单数。

比如：What I am most interested in is American movies．

但是，OG211也可被改为‘what are much more difficult to determine are’，因为这个选择没有出现在选项中，所以B是优选；另外，从本题看来，ETS更倾向于用单数。