[单选题]
A boat can travel 4.0 m/s in sgz6b + fi1vx/ftill water. With what speed, .l(gyn ws4 .y9lccf i7p1+rayrelative to the shore, does it move in a river that is flowing p1 74 cy+yf9lyri cnw.gla(s.at 1.0 m/s if the boat is heading straight across the river?