题目内容：

Is this problem ___ the teacher explained last week?
A which
B that
C the one
D one
我选B老师打错！谁能讲的清楚点。
我语法不好

优质解答

C.
换为陈述句看：
this problem is the one the teacher explained last week.
this problem作了explain的宾语,特指the teacher解释的问题,所以是the one.