27. The boy was busy doing his homework onlyonce in a while to look out of the window.A.

问题补充：

27. The boy was busy doing his homework, onlyonce in a while to look out of the window.

A. stopping

B. to stop

C. stopped

D. having stopped

答案：

(答案→)A. stopping

B. to stop

C. stopped

D. having stopped

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