In just three years, the Net has gone from a playground for the local people to a vast communications and trading center where millions swap information or do deals around the world.

A．business

B．shopping

C．chatting

D．meeting

引起肺心病的关键环节是（）

A.小气道阻塞

B.肺部感染

C.肺动脉高压

D.肺间质纤维组织增生

E.肺水肿