Only when according to the directions can the medicine be quite effective.
A. taking
B. taken
C. being taken
D. having been taken
网友回答
A. taking
(答案→)B. taken
C. being taken
D. having been taken
解析：句意：只有当按照说明被服用这个药才能非常有效。这里用了状语从句的省略，完整的是only when the medicine is taken according to the directions can…，当主从句的主语一致的时候，可以省略从句的主语和be动词，所以选B。