OG SC 24 以前不觉得奇怪现在觉得奇怪

The period when the great painted caves at Lascaux and Altamira 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

答案是B

我奇怪为什么用单数呢？

what is much more difficult to determine

这是个主语从句，谓语肯定用单数。与宾语无关。