题目内容
______, it began to run unexpectedly.
A. While cleaning the machine B. To clean the machine
C. He was cleaning the machine D. While he was cleaning the machine
D
如果用现在分词作状语,其逻辑主语应与句子主语保持一致。A项不符合这一要求;C项与后一分句之间缺少连接词。
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