The World Health Organization warned that Asian countries __________ attentive to bird flu since Asia ___________ got rid of the disease completely.

A．should keep; has

B．must remain; has not

C．ought to stay; has been

D．shall be; has not been

司机开车上坡前，往往加大油门，以提高汽车的速度，这是为了[ ]

A、增大动能

B、增大势能

C、增大惯性

D、减少阻力