-Look at the boy running on the playground. Is it David? ---It be him. I saw him go to

问题补充：

---Look at the boy running on the playground. Is it David?

---It be him. I saw him go to the teachers’ office just now.

A. mustn’t

B. can’t

C. couldn’t

D. must

答案：

A. mustn’t

(答案→)B. can’t

C. couldn’t

D. must

解析：考查情态动词表推测的用法。表示否定性的推测要用can’t。肯定推测用must，mustn’t表示禁止，不允许，couldn’t是can’t的过去式，句意：看在操场跑的那个男孩，是大卫吗？一定不是他，我刚才看到他去了老师的办公室。所以选B。