Question

Look at your face. It's dirty. You ______ wash your face.

1. A.
   must
2. B.
   have to
3. C.
   mustn't
4. D.
   don't have to

Answer 1

B
解析：
脸脏是客观上的外部原因，因为脸很脏所以非得要洗，所以用have to，故B。