it 做形式主语引导的主语从句和强调句型的区别

it 做形式主语引导的主语从句和强调句型的区别
比如
Tom thought he must have sen a strange insect ,but it was the goat's eyes ___ he had seen in darkness.
A.which B.that C.what D.when
这题选什么、
but it was the goat's eyes ___ he had seen in darkness.
这是个强调句型吗?我为什么看不出来它和it 做形式主语引导的主语从句有什么区别.
it 做形式主语引导的主语从句和强调句型怎么区分啊.
说明一下为什么不选B？