They were busy ________ an experiment when I came.A．doingB．to be doingC．with doi_刷题宝

题干

They were busy ________ an experiment when I came.

A．doing

B．to be doing

C．with doing

D．to do

上一题下一题 0.64难度单项选择更新时间:2013-04-10 09:32:21

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