Many cities issued a warning of         air pollution for PM-2.5 readings, which became

        Concern of all the people

A．the; the

B．the;/

C．/;a

D．an;a

暑假小明和朋友一起去水帘洞游玩，他们在水中划船时，使船前进的动力是（　　）

A．船的重力

B．水直接对船身的推力

C．人对船的推力

D．水对桨的推力