高中英语定语从句,

高中英语定语从句,
这5个题的答案是AADBB.为什么选这个,为什么排除其他答案.
1.Don’t talk about such things of __________you are not sure.
A.which
B.what
C.as
D.those
2.Is this the factory __________ you visited the other day?
A.that
B.where
C.in which
D.the one
3.Is this factory __________ some foreign friends visited last Friday?
A.that
B.where
C.which
D.the one
4.Is this the factory __________ he worked ten years ago?
A.that
B.where
C.which
D.the one
7.This book will show you __________ can beused in other contexts..
A.how you have observed
B.what you have observed
C.that you have observed
D.how that you have observed