When ____ medicine, please follow these instructions carefully.

When ____ medicine, please follow these instructions carefully.
A、to take
B、taking
C、taken
D、he takes
【正确答案】：B
【题目解析】：【考点点击】本题考查状语从句的省略。【要点透析】状语从句中省去了从句的主语,而主句为祈使句,主语为you;主语you与take之间为主动关系,因此用现在分词。