that he said so must be quite encouraging,i think

题目

that he said so must be quite encouraging,i think
这里为什么不用what来引导,而用that
是把so 当作said 的宾语吗
好象没有这个用法吧

答案

he said so是个完整的句子,所以应该that引导
如果没so就应该what
()