Even the best student in our class couldn't answer this question, so it ________ be very difficult．

1. A.
   must
2. B.
   could
3. C.
   may
4. D.
   can

Even the best student in our class couldn't answer this question, so it ________ be very difficult.

A. must

B. could

C. may

D. can

Even the best student in our class couldn't answer this question，so it _________ be very difficult．

A．must          B．could              C．may          D．can