OG VERBAL SC 24

The perid 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

D

答案是B，但是哪个more difficult to determine 的不是有the reason, the use,the meaning三个吗？what引导的名词从句要以意思定单复数吧？我这么推有什么错吗？请大家指教 ：）

了解。。。。然后突然觉得自己特傻，呵呵。。。有CD真好~