He was busy writing the exam paper only once in a while to check the time. A. to stopB.

问题补充：

He was busy writing the exam paper , only once in a while to check the time.

A. to stop

B. stopping

C. to have stopped

D. having stopped

答案：

A. to stop

(答案→)B. stopping

C. to have stopped

D. having stopped

解析：考查分词做状语。He与stop构成主动关系，使用现在分词的形式。句意：他忙于做试卷，只是偶尔停下来核对一下时间。