________ people go abroad, they have to bring their passports with them.

A．Until

B．When

C．Because

D．Before

低氧性肺血管收缩是指（）。

A.肺泡低氧引起的肺小静脉收缩

B.肺泡高二氧化碳引起的肺动脉收缩

C.肺泡高二氧化碳引起的肺静脉收缩

D.肺泡低氧引起的肺小动脉收缩

E.肺泡低氧引起的肺支气管收缩