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：考查非限制性定语从句。句意：居住在山区有许多问题，找到水源就是其中一个很大的难题。what不用于定语从句，whose作定语，后常跟一个名词，排除B、D；as引导非限制性定语从句，作主语或宾语，这里不缺少成分，排除C；obtaining water和problems是从属关系，应该用of which引导定语从句，故选A。