A man can swim with a speed of 4 kmh⁻¹ in still water. He crosses a river 1 km wide that flows steadily at 3 kmh⁻¹. If he makes his strokes normal to the river current, how far down the river does he go when he reaches the other bank?
a. 500 m
b. 600 m
c. 750 m
d. 850 m