一个心动周期是指（）。
A、心脏每收缩一次
B、心脏每舒张一次
C、心脏每收缩和舒张一次
D、不一定
参考答案：C
Without facts, we can't form a worthwhile opinion for we need to have factual knowledge _____ our thinking. [ ]
A. upon which to base
B. which to be based on
C. which to base on
D. to which to be based on
CO2气体保护焊，焊接回路中串联电感，可以调节（），达到控制熔深的目的。
A、电弧燃烧时间
B、短路电流增长速度
C、焊接电流
D、焊接速度
