Generally speaking, when according to the directions, the medicine has no side effect.
A. taken
B. taking
C. to take
D. to be taken
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(答案→)A. taken
B. taking
C. to take
D. to be taken
解析：考查状语从句的省略：句意：“通常说来，当被按照说明书服用时，这个要没有副作用。”应为这句话的主语是drug，所以应该是“被服用”，所以是when +过去分词。省略了the medicine is，选A。