期前收缩的产生机制包括()
A.附加的房室传导束
B.迷走神经张力增高
C.折返激动
D.触发活动
E.异位起搏点的兴奋性增高
参考答案：C, D, E
It never occurred to me _____ you could succeed in per- suading him to change his mind.
[ ]
A. which
B. what
C. that
D. if
槽形透度计的灵敏度计算按（）（X为可发现的最浅槽深；A是透度计总厚度；T为被透工件厚度）
A、K=（T/X）100%
B、（X/T）100%
C、[X/（T+A）]100%
D、以上a和c
