问题
选择题
______ 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
答案
D
题目分析：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。