The doctor was busy ____ on the boy at that time.B.to operate D.operatingB为什么错了?

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The doctor was busy ____ on the boy at that time.
B.to operate D.operating
B为什么错了?

be busy doing sth是固定词组

be busy doing是 固定搭配哦

be busy doing sth 忙于做某事

.固定词组~~~