A Horse Exerts 500 N of Force on a Heavy

Multiple Choice

A horse exerts 500 N of force on a heavy wagon. The wagon pulls back on the horse with an equal force. The wagon still accelerates because

A) these forces are not an action-reaction pair.
B) the wagon does not accelerate because these forces are equal and opposite.
C) the horse pulls on the wagon a brief time before the wagon reacts.
D) nevertheless there is still an unbalanced force on the wagon.
E) the wagon is not alive.