---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：根据后文的内容，可知为否定的选择为B