心肌自律细胞的自律性高低主要取决于动作电位的( )。
A. 0期去极化速度快慢
B. 1期复极化速度快慢
C. 舒张期自动去极化速度快慢
D. 快离子通道和慢离子通道的比例
E. 以上都不是
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通过关联关键字“系别”这一相同字段，表二和表一构成的关系为( )。
A. 一对一
B. 多对一
C. 一对多
D. 多对多
实验设计时设立对照组的主要目的之一是（）。
A. 操作自变量
B. 测量因变量
C. 控制无关变量
D. 增加实验被试
One explanation given by scientists for the crop circles is that they are made by ______.
A. airplane crashes
B. air movement
C. unknown flying objects
D. new farming techniques
Which of the following may prove that the crop circles are not made by man
A. The farmers couldn't step out of the field
B. The farmers couldn't make the circles round
C. The farmers couldn't leave without footprints
D. The farmers couldn't keep the wheat straight up
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