I don’t know .A. what to do it B. how to do it C. what to do D. to do what

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A. what to do it
B. how to do it
(答案→)C. what to do
D. to do what
解析：考查疑问词+不定式。what to do中，what作do的宾语，A项it多余，排除A；D项的疑问词应该放前面，排除D。 how to do中缺少宾语，排除B。故选C。