单项选择题

A.汽水同向流动时25m/s;汽水逆向流动时20m/s
B.汽水同向流动时30m/s;汽水逆向流动时20m/s
C.汽水同向流动时25m/s;汽水逆向流动时30m/s
D.汽水同向流动时30m/s;汽水逆向流动时25m/s