Living in the mountain area has its problems, ________ obtaining water is a big problem.
#66944
A
of which
B
for what
C
as
D
whose
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I have two grammars, ______are of great use. 
#73806
A
all of which
B
either of which
C
both of that
D
both of which
 It was quite a serious accident, _____ caused by careless driving.
#72311
A
which I think was
B
I think which was
C
which I think it was
D
I think which it was