Generally speaking , according to the directions , the drug has no side effect .A. when ta

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A. when taking
(答案→)B. when taken
C. when to take
D. when to be taken
解析：当分词的逻辑主语也就是主句的主语，这时可以省去逻辑主语和be动词.现在分词表示主动和进行，而过去分词表示被动和完成。在这个句子中，其实是when the drug is taken,省去了the drug is，the drug 和take是被动关系．