For years, it was viewed as a serious problem, that has never been really solved.
A. one
B. where
C. it
D. which
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(答案→)A. one
B. where
C. it
D. which
解析：句意:多年来,它被视为一个严重的问题,一个从来没有真正解决。one是a serious problem 的同位语，代指前面出现的单数名词。Where在哪里，先行词应为地点名词；it代指具体唯一的事物；因此排除B C两项。which引导定语从句，与that重复，故排除。因此选A。