英语求教,我有答案请告诉我为什么这样选

题目

英语求教,我有答案请告诉我为什么这样选
9.This is _____ I can do for you right now.
A.which B.that C.what D.as
10.It was at our college library _____ I borrowed the novel.
A.which B.in which C.that D.where
11.I have seen trees,_____ open at sunrise and close at sunset.
A.which the leaves B.of which leaves C.whose leaves D.its leaves

答案