They were busy an experiment when I came.A. doingB. to be doingC. with doingD. to do

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(答案→)A. doing
B. to be doing
C. with doing
D. to do
解析：考查固定搭配。固定搭配：be busy doing sth忙于做某事；be busy with sth忙于某事；句意：当我来的时候，他们忙于做实验。故A正确。