______ has an internal clock that regulates the beating of our hearts, and the ______ of our breathingA.Mankind; stepB.Human; paceC.Human being; stepD.Man; pace
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D解析试题分析：考查名词：句意：人类有永恒的时鈡（生物钟），它控制着我们的心跳和呼吸的频率。human 或human being是可数名词，可以排除， pace是步幅，频率 而steps是脚步，选D。考点：考查名词