The clever boy made a hole in the wall,__he could see what w

原题应该是这样的吧?!The clever boy made a hole in the wall,__ he could see __ was going inside the house.(你把第二空需要填的词what已经写出来了)
答案:B.through which,what
这儿考查定语从句,由于在介词之后,故应该用which引导定语从句,再结合意思"透过这个洞他能看到..."可知介词要用through,故答案选B.第二空是what引导的宾语从句作could see的宾语,what在宾语从句中作主语.
因为介词后不能用that引导定语从句,因此C不正确.
句意:那个聪明的男孩在墙上弄了个洞,透过这个洞他能看到房子里面的情况.