The boat is headed with a velocity of 18 m/seconds toward the west with respect to the water in a river. If the river is flowing with a velocity of 2.5 m/s in the same direction as the boat, what would be the magnitude of the boat's velocity?
a) 20.5 m/s
b) 15.5 m/s
c) 16.5 m/s
d) 22.5 m/s