So quickly are science and technology advancing ________ is a possibility today may be a reality tomorrow.

A．which

B．what

C．that which

D．that what

窦性心动过速（）。心动过速者按压颈动脉窦后，心率可产生以下变化

A．突然终止

B．无变化

C．频率渐缓

D．频率减至原心率的一半

E．频率加速