请教Prep中的一道语法题

tracyforever · 发表于 2010-3-20 12:35:38

The period when the great painted caves at Lascanx and Arlamira were occupied by upper Poleolithic peoples has been established by canbon 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 magnificatly depicted animals.

D) has been established by carbon dating, but what is much more difficult to determine is

为什么这里用的是is?后面的非划线部分不是还有3个并列的宾语么？但是根据前面的句型，what is much more difficult to determine 后面应该是要跟is的。。。
请教大虾，是我理解错误么？

Bourne_NY · 发表于 2010-3-20 20:35:37

系动词的数是跟主语的，what引导的主语从句要求系动词用is，不用管后面表语或者宾语。

请教Prep中的一道语法题

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