turned out that an army officer set off the bombs．A. WhatB. ItC. TheyD. As

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A. What
(答案→)B. It
C. They
D. As
解析：考查主语从句。It常用来作形式主语，真正的主语是后面的that引导的主语从句。Turn out“结果是，证明是”；句意“结果证明是一个军官发射的炸弹。”故选B。