he broke down at last , i think it was_____ he had overworked. A the reason B that C why D because

he broke down at last , i think it was_____ he had overworked. A the reason B that C why D because

because 引导表语从句。并且它的主语只能是it /this/that.
用A the reason语意不通。那就是他过度工作的理由。显然不合适。应该改为他过度工作是他身体垮下来的原因。
用B that必需用the reason做主语。 he broke down at last ,the reason is that he had overworked.
用why 表示，，，的原因。也不合语意。那就是他过度工作的原因。应该改为那就是他倒下的原因。即 that he had overworked was why he broke down at last.

it指代he broke down at last（他最终累垮了） i think是插入语
他的身体最终跨了，因为他过度工作

他的身体最终跨了，我想是因为他过度工作造成的。在这里是要填一个表原因的，所以排除B, A .the reaon 一般与why 搭配使用，C.why 是为什么的意思，我们翻译一下，他身体最终垮了，我猜这是为什么他工作过度，明显放这里翻译不通。

选D，他身体垮掉了，我认为这是因为他工作过度了。
because 除经常用于引导原因状语从句外，还可引出表语从句或用于强调句等。如：
It is because you’re eating too much. 那是因为你吃得太多了。
It was because I wanted to buy a dictionary that I went to town yesterday. 我昨天是由于想买本字典而进城的。

这是一个结果放到前面的句子，he broke down at last“他最终累垮了”，后面就是原因：i think it was_____ he had overworked. 我觉得他是劳累过度