下列哪项可引起心率减慢（）
A.交感神经活动增强
B.迷走神经活动增强
C.肾上腺素
D.甲状腺激素
E.发热
参考答案：B
---I heard that Bill was scolded by the teacher last week.
---Ah, yes. The teacher wanted to check his exercises, but ___ was finished at all.
A．nothing
B．either
C．neither
D．none
试分析试题3-30图所示电路为（）。
A．单稳态触发器
B．多谐振荡器
C．RS触发器
D．施密特触发器
