Living in Western China certainly has its problems, obtaining water is not the least.
A. of which
B. where
C. as
D. whose
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(答案→)A. of which
B. where
C. as
D. whose
解析：考查定语从句。句中先行词为problem，在定语从句作obtaining water of的宾语，故用关系词which。句意：生活在中国西部有它一定的问题，其中重要的一个就是取水问题。