Question

---How unhappy your roommate looks!
---Yes, but he's not willing to tell me what it is _______ is troubling him.

1. A.
   what
2. B.
   which
3. C.
   as
4. D.
   that

Answer 1

D
强调句型的特殊疑问式，相当于对“is troubling him”的主语进行强调然后再提问。