You can't imagine the difficulty they had [ ] the problem.It isn't [ ] easy.A.to solve; soB.solved; soC.solving; thatD.solved; that为什么?

此题答案选C，可看作是现在分词solving作时间状语，有时可用“when(或while)+现在分词”结构强调分词所表示的动作与谓语动词所表示的动作同时发生。所以这句话还可以写成You can't imagine the difficulty they had when they solved the problem. It isn't that easy.译为：你想象不到他们解决问题时的难度，不是那么容易的。后面的that相当于一个程度副词。

solving the pro形容的是difficulty ，they had是插入语，可省略。“that＋形象词” 表示“那么…”，是固定用法

have difficulty (in) doing something,固定搭配,只能选C
that,作“那么”解
语感吧,多读读英文

B,完成时做后置定语