You ____ ____ leave rubblish in the playground.
下列运算正确的是( )
①燕子是候鸟,能长途飞行,怎么还用专列送呢?
②她的脸冻得通红,手冻得僵硬,但她一点也不在乎。
呼吸道感染时,常并发中耳炎,其病菌的传播通道是 ( )
He often ____ (go) fishing after class.