These compounds could not have been formed if the chemical reaction ______ stopped.

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问题 These compounds could not have been formed if the chemical reaction ______ stopped.

选项 A、have been
B、has been
C、had been
D、were

答案C

解析虚拟语气题。主句中是 could have done，相对应的 if 引导的条件状语从句中的时态应为过去完成时 had done，是对过去已发生事情的虚拟。本句大意为：若是当时化学反应被中止，这些化合物就无法形成。

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