应该填why
why=for which
所以选B
...the reason why ...
结构∶...the reason + why ...
说明∶此句型意为“…的理由”.why 是,由“+ which”变化而成,引导,修饰.可保留 why,而将前面的省略;也可将 why 省略.the reason why 无非限定用法,即 why 之前不可以加逗点.
Please tell me the reason why (=for which) you were absent.请告诉我你为什麼缺席.
Tell me the reason (why) he refused to listen to my advice.告诉我为何他拒绝听从我的劝告.
That’s (the reason) why he cried.那就是他为何哭的原因.