疑问：语法笔记中的一题

ayoayo

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

句子后半段的the use...the reason...and the meaning 这三个是算单数还是合起来是个复数？
还是因为前面是用what is 所以后面也用is？

ayoayo

木有人啊木有人

skyemotion

the use to which primitive peoples put the caves, the reason for their decoration, and the meaning of the magnificently depicted animals.
三个成分并列作为　what is much more difficult to determine　的宾语。