问题补充：
Living in the central Australian desert has its problems, obtaining water is not the least.
A. for which
B. to which
C. of which
D. in which
答案：
A. for which
B. to which
(答案→)C. of which
D. in which
解析：考查定语从句中介词与关系代词的用法。先行词为problems,此句意思为“在这些问题中获得水是最重要的”。在---中，因此选C
