[单选题]
A boat can travel 4.0 m/s in still+yvckdvm3kuduhg z2 0+:)n)i wat3 kjf8x-.vz7 fkff1phay ae+8er. With what speed, relative to the shore, k783a kxv1j.-fafe+f 8y phf zdoes it move in a river that is flowing at 1.0 m/s if the boat is heading upstream?
