we will start the project has not been decided yet.A. ThatB. WhoC. WhatD. When

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A. That
B. Who
C. What
(答案→)D. When
解析：句意：我们何时开始这个计划还没有决定。这里是when引导的主语从句，它在从句中作状语，that在主语从句中不作成分，主句中不缺少who的词义，what在从句中作主语和宾语，这里不缺少，故选D。