该用is 还是are？求大神解答

Eve001

Ron曾经讲到这道题，what is more difficult to determine.... 这个句子的谓语单复数应该根据后面的部分来确定，也就是the use to which......animals. 当时Ron给出的答案是a，为什么我今天做gmart prep的时候遇到这道题，官方答案给的是d. 疑惑中，还请大神解答
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 is much more difficult to determine are

b. have been established by carbon dating, but what is much more difficult to determine are

c. have been established by carbon dating, but that which is much more difficult to determine is

d. has been established by carbon dating, but what is much more difficult to determine is

e. are established by carbon dating, but that which is much more difficult to determine is

Sebastiann

肯定是A
这样想：

这句句子简化来看就是
A are X,Y and Z

lzc95232

楼主，明确告诉你记错了， the hardest part of the test (was/were) the last three questions, 填 was 还是were ? 情况和这题一样

GGininder

這題的has 沒有問題，因為主詞是the period ... has ...
, but 後面為完整句子，所以主詞為 what is much more difficult to determine，那麼動詞要接單數還是複數呢？

Jasmine0_0

楼主，你记错了哈，我刚看的视频，答案就是d。RON说的是what is much more difficult to determine是主语，下面是Ron原话：
IF anything before the verb can be the subject，then that‘s the subject.
If nothing before the verb can be the subject,then you have a backward construction
what是pronoun，可以做主语。所以这句话不属于倒装结构，谓语动词is后面的那一堆不能做主语。