---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

---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

—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

--- 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