When he walks my desk he often knocks my books the desk.A. pass; offB. past; offC. pas

问题补充：

When he walks my desk, he often knocks my books the desk.

A. pass; off

B. past; off

C. pass; on

D. past; on

答案：

A. pass; off

(答案→)B. past; off

C. pass; on

D. past; on

解析：pass为动词，而第一空之前的词为动词，故选介词，所以将BC排除，knock sth on sth意思为用····敲···，为固定词组搭配。故选答案为B