题目内容：

[高手进~]英语.伴随情况状语
高手速度看过题后,麻烦给讲下作题思路（要详细呦~）!~先谢啦,呵呵呵~
He was busy writting a story, only ___ once in a while
to smoke a cigarette.
A.to stop
B.stopping
C.to have stop
D.having stopping