has an internal clock that regulates the beating of our hearts, and the of our breathing.
A. Mankind; step
B. Human; pace
C. Human being; step
D. Man; pace
网友回答
A. Mankind; step
B. Human; pace
C. Human being; step
(答案→)D. Man; pace
解析：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。