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

问题补充：

They were busy an experiment when I came.

A. doing

B. to be doing

C. with doing

D. to do

答案：

(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。