Please_____hesitate to ask me if you have any problems.

Please_____hesitate to ask me if you have any problems.
A. not to.
B. do not.
C. not.
D. no.
      满分：2  分
2.  Someone is at the door. _____must be Mr. Black.
A. He.
B. It.
C. This.
D. That. A
      满分：2  分
3.  Group discussion_____a good opportunity for students to practice oral Eng¬lish.
A. process.
B. proves.
C. progress.