问题
选择题
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
答案：B
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。
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