问一个简单的问题 大全211

211.
            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（B）

(E) are established by carbon-14 dating, but that which is much more difficult to determine is

答案是B.

what is much more difficult to determine is the reason for their decoration, the use and the meaning.

这里reason是单数，后面却跟了三个内容，属于合理的语法现象吗？

我理解错了。 原来是is 后面跟了三个宾语。