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

來源：國語幫 1.95W

— 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　這是一個強調句型用於強調what引導的賓語從句。去掉it is that，句子成分仍然完整。