【题目】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】B
【解析】句意：他忙于做试卷，只是偶尔停下来核对一下时间。He与stop构成主动关系，使用现在分词的形式，故选B。