A car pushes a stalled truck up a hill. The car exerts a force of magnitude F1 on the truck. The truck exerts a force of magnitude F2 on the car. Then
(a) F1=F2 since these forces form an action-reaction pair.
(b) F1=F2 only if the car and truck do not accelerate.
(c) F1>F2, or else the car and the truck would not move.
(d) F1<F2 because the truck is heavier.