A boat can be rowed at a speed of 8.15 km/h in still water. The boat is in a river that moves at a speed of 2.53 km/h. The river is 0.735 km wide and flows in a straight line with straight banks. What is the resultant velocity of the boat when rowing upstream?

a) 5.62 km/h
b) 6.62 km/h
c) 7.68 km/h
d) 9.68 km/h