四川成都平原素有“天府之国”的美称，从历史上看，使成都平原变成“天府之国”的美称的重要水利工程是（　　）

A．郑国渠

B．都江堰

C．灵渠

D．六辅渠

. My pain ___________ obvious the moment I walked into the room, for the first man I met  asked me pitifully:" Are you feeling all right?"

A．must be

B．must have been

C．could be

D．could have been