沿河设城主要是因为

①河流为城市提供生活用水和工业用水    ②河流为城市运送废水，加速净化

③河流为城市提供便利的水运条件        ④河流为城市增加一条风景线，并有利于防卫

A．①②

B．①③

C．①④

D．②④

The speed of a vessel adjusted to that of a pilot boat at which the pilot can safely embark is（）.

A.ground speed

B.boarding speed

C.relative speed

D.speed of the vessel