For years it was viewed as a serious problem that has never been really solved.A. oneB.

问题补充：

For years, it was viewed as a serious problem, that has never been really solved.

A. one

B. where

C. it

D. which

答案：

(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。