求助英语选择题2what he couldn't understand was why more and more yo

求助英语选择题2
what he couldn't understand was why more and more young people like going abroad .
A、What ; why B、That ; what C、What ; because D、Why ; that
请详细解释 谢谢
不好意思刚才出错了现在从新写一遍题
________ he couldn't understand was_______more and more young people like going abroad .
A、What ; why B、That ; what C、What ; because D、Why ; that
谢谢你的回答。
我也认为选A 可是我不明白的是why 引导的是宾语从句了whathe couldn't understand 译成他不明白的是
是什么语法现象？引导的是什么从句