问题补充：
Generally speaking, according to the directions, the drug has no side effect.
A. when taking
B. when taken
C. when to take
D. when to be taken
答案：
A. when taking
(答案→)B. when taken
C. when to take
D. when to be taken
解析：状语从句的省略。补充完整是when the drug is taken according to the directions,可以省略the drug is。选B。
