Passage Three

What would happen to some countries if tourism stopped

A．Their economies would be ruined.

B．It would keep their economies going.

C．There would be no more environmental pollution there.

D．People have no chance to travel there.

能够保护中枢系统，选择性摄取外来物质（）

A.表观分布容积

B.血脑屏障

C.肝脏

D.肝肠循环

E.肾脏