Nobody told us A. what to do it B. how to do C. where to do D. when to do it

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A. what to do it
B. how to do
C. where to do
(答案→)D. when to do it
解析：考查疑问词+不定式的用法。What作do的宾语，it多余，排除A；B、C两项缺少宾语；D项不缺宾语。句意“没有人告诉我们什么时候做这件事。”故选D。