英语语法问题,关于复合句的

题目

英语语法问题,关于复合句的
In the A.D.300's,a church was built over _______ was thought to be Saint Perter's Tomb.
这里答案是what.有点不能理解.where的话我想很多同学都会填这个的吧.我首先来分析下,what was thought .做的是over 的宾语,而what既做over的宾语,也做它引导的宾语从句的主语.
但是我看了下语法书,where应该也有同样的功能吧,意思是没有看到可以排除where的语法,可能是我看漏了,但是找了几次的确没有找到.

答案

In the A.D.300's, a church was built over _______ was thought to be Saint Perter's Tomb.
In the A.D.300's, a church was built over【what】 was thought to be Saint Perter's Tomb.
应该填what.因为what=the thing which/ that.