A car of mass m travelling with a uniform speed v crosses over a bridge which is in the form of a convex arc of radius r. When the car is at the middle point of the bridge, then the force exerted by the car on the bridge is:

a. mg
b. mg + (r * mv²) / 2
c. mg - (r * mv²) / 2
d. mg - (r² * mv²) / 2