it began to rain unexpectedly.A. While cleaning the machineB. To clean the machineC. He

问题补充：

, it began to rain unexpectedly.

A. While cleaning the machine

B. To clean the machine

C. He was cleaning the machine

D. While he was cleaning the machine

答案：

A. While cleaning the machine

B. To clean the machine

C. He was cleaning the machine

(答案→)D. While he was cleaning the machine

解析：考查句子结构。后面句子的主语是it说明AB错误，因为要使用非谓语动词做状语，要求非谓语动词的逻辑主语是句子的主语。C项是一个完整的句子，逗号后面也是完整的句子，两个句子之间缺少连词连接，故C错误。句意：当他在清洁机器的时候，突然开始下起雨了。故D正确。