—Howunhappyyourroommatelooks!—Yes,buthe'snotwillingtote...

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— How unhappy your roommate looks!

— Yes, but he's not willing to tell me what it is ________ is troubling him.

A. what B. which

C. as D. that

D　这是一个强调句型用于强调what引导的宾语从句。去掉it is that，句子成分仍然完整。