________ is known to everybody, the impact of the Iraq war on the United States and Iraq is so great that it may take years for people in both countries to recover from it.A.SoB.AsC.ItD.That
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D解析这是一个含有非限制性定语从句的主从复合句，关系代词as代表主句的整个内容，在从句中作主语。如果用It作形式主语，作真正主语的主语从句置于句子后部，应把逗号改成that。