Living in the mountain area has its problems , obtaining water is a big problem.
A. of which
B. for what
C. as
D. whose
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(答案→)A. of which
B. for what
C. as
D. whose
解析：句意：生活在山区有它的问题，其中取水是个很大的问题。这里是非限制性定语从句，先行词是problems，表示在众多问题当中的一个，，表示整体和部分的关系，用介词of，所以用A