关于“what引导主语从句的主谓一致”的一点理解

那determine后面不是并列了三个很难决定的东西吗？这三个东西不就是主句，所以谓语动词应该用ARE。从而what后面的谓语动词应该与主句的一致，所以用what are。可是为什么这题全用的是IS。我从来没有见过what are的例子。

请NN们帮忙解答下！谢谢啦

之前一直记得是，what引导主语从句，谓语用单数，但是今天看了08Prep语法笔记后发现还是有点复杂的。。。

166.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

语法笔记的解答有这么一段话：
1.what引导的名词性从句中从句谓语动词的单复数情况：
what后面“什么时候用单数,什么时候用复数”根据动词后面的名词的单复数来定.

2.what引导的名词性从句做主语时，主句谓语动词的单复数情况：
(1)What在从句中做主语，主句谓语由what从句的谓语决定。
What is difficult to understand is A.
What are difficult to understand are A and B.
(2)what在从句中做宾语，主句谓语由主句表语/宾语决定what they want are XX,XX&XX。
What his father left him are a few English books.

也就是说，what引导主语从句，主句的主谓一致，不一定要用is，而是what后面用啥主句就用啥: what is... is... /what are...are...
而what从句中的主谓一致，位于的单复数则需要根据主句动词后面名词单复数来决定
所以，结合以上两点才会有 What are difficult to understand are A and B

在第二条规则里的后两个例子中，what they want are A, B, and C. what his father left him are ...books. 个人感觉应该是倒装，把宾语提前了

不太明白LZ的意思，为什么说这两个例子是宾语提前呢？我感觉这两句表达很正常啊，没倒装。