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
网友回答
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。