He set off his missing dog.A. in search forB. search forC. search ofD. in search of

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A. in search for
B. search for
C. search of
(答案→)D. in search of
解析： 考查短语：search for 和 in search of为了寻找什么，题中已经有了谓语动词，故用介词短语作状语，句意：他出发了为了寻找他失踪的狗。故选D。