主要用于解释短气隙、低气压的气体放电的是（）理论。
A.电子碰撞电离
B.流注
C.小桥
D.汤逊
参考答案：D
环境人口容量具有[ ]
A、不确定性和绝对稳定性
B、确定性和绝对稳定性
C、不确定性和相对确定性
D、不稳定性和不确定性
When we _______the railway station, the train _______for 5 minutes．
A．got to, had been away
B．reached, had left
C．arrived, has been away
D．arrived at, has left
