心室收缩时关闭的是（）
A.主动脉瓣和肺动脉瓣
B.二尖瓣和三尖瓣
C.主动脉瓣和二尖瓣
D.肺动脉瓣和三尖瓣
E.主动脉瓣和三尖瓣
参考答案：B
The serious situation made it possible for laws preventing pollutions_________in many countries.
A．from passing
B．to pass
C．by passing
D．to be passed
汽车上原有50人，到某站下车x人，又上来y人，现在车上有______人．
