[单选题]
A boat can travel 4.0 m/s in still water. With what speed, relative to the shj47i*( kslfoy orkoifs*j4 yl(7 a53tmxm4dg p* l+2skbn 87wgje, does it move in a river that is flowing at 1.0 m/s if skgap7 l *3d 45ntjx8mbg2+ wmthe boat is heading straight across the river?
