题目

-Look! The man at the gate            be our teacher. He is always  standing there every morning.          -No, it          be him. He is having a meeting in the office now.         A. must; can't       B. must; mustn't       C. can't; can't    D. can; couldn't 答案：A. 一个物体向东运动了300 m，又向南运动了400 m，用时25s，求物体的平均速度