[单选题]
A boat can travel 4.0 m/s in still water. With what speed, relati s8*yv 5vbwy r)fuf ++e,k*hmlve to f+5o kr7 -s..vkqmhw7 c*vkpm the shore, does it move in a river that is flowing at 1.0 m/s if the boat is headiv rcm .h.s7*kwfqp k-7 +vk5mong upstream?
