Mr. Smiths was busy writing a story, only once in a while to smoke a cigarette.
A. to stop
B. to have stopped
C. being stopped
D. stopping
网友回答
A. to stop
B. to have stopped
C. being stopped
(答案→)D. stopping
解析：句意：史密斯先生忙于写故事，只偶尔停下来抽一根烟。Stop和逻辑主语Mr. Smiths 是主谓关系，故用现在分词作伴随状语。故选D。