Question

The fact _______ we were interested in____all of them. A.that;was surprised B.which;surprised

为什么选B不选A呢？为什么用which?

Answer 1

此处which是做in的宾语, 相当于 we were interested in which. 当做介词后的宾语用时，只能用which/what,而不能用that/how/when/where.
其实此处省略去了一个that，做同位语从句引导后面的which we were interested in，做surprised all of us 的主语。The fact (that) which we were interested in .... the fact 即相当于which we were interested in, that 额可以省去。