28. ---- Who told you my email address?

   ---- I don’t remember clearly. It _______ Karen.

   ---- It _______ Karen. She doesn’t know my email address.

A. may have been; can’t have been           B. must have been; mustn’t have been

C. may have been; must have been           D. must have been; mustn’t be 

--- Who told you my email address?
--- I don’t remember clearly. It ______ Karen.
--- It ______ Karen. She doesn’t know my email address.

1. A.
   may have been; can’t have been
2. B.
   must have been; mustn’t have been
3. C.
   may have been; must have been
4. D.
   must have been; mustn’t be