单项选择题

A.汽水同向流动时80m/s,汽水逆向流动时60m/s
B.汽水同向流动时60m/s,汽水逆向流动时80m/s
C.汽水同向流动时80m/s,汽水逆向流动时50m/s
D.汽水同向流动时50m/s,汽水逆向流动时80m/s