求助!大全_17_22

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

为何选B,为什么用is而不用are,求助各位Nn解释一下what做主语的用法,谢谢!

发错地方了吧，麻烦版主转到语法版。

晕，mm你没划线线，我给你凑合解释一下吧。

has的主语是period, is的主语是后面的the reason,明白否？

mm如果是初学的话，建议先看语法书，再做题。such as 白勇、刘振民