Only according to the directions can the medicine be quite effective.A. takingB. takenC. b

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：考查分词用法。Medicine与take之间构成被动关系，所以使用过去分词的形式，C表示正在被服用；D表示已经被服用；句意：只有根据说明书服用，这个要才相当有效。