问题补充：
Only according to the directions can the medicine be quite effective.
A. taking
B. taken
C. being taken
D. having been taken
答案：
A. taking
(答案→)B. taken
C. being taken
D. having been taken
解析：选项动词的逻辑主语the medicine与动词take之间为被动关系，实际上相当于一个时间状语从句only when it is taken...结构，故应选taken在句中用作状语。
