If action is always equal to the reaction,explain,why a cart pulled by a horse can be moved.

(D) The motion of the horse and cart system is governed by the following forces:
$(i)$ The weight of the horse and cart acting vertically downward.
$(ii)$ The normal reaction of the ground acting vertically upward,which balances the weight.
$(iii)$ The force exerted by the horse's feet on the ground in the backward direction (Action).
$(iv)$ The reaction force of the ground on the horse's feet in the forward direction (Reaction).
$(v)$ The force of friction acting on the cart wheels in the backward direction.
Forces $(i)$ and $(ii)$ are balanced and do not contribute to horizontal motion. The horse pushes the ground backward with force $(iii)$,and the ground exerts an equal and opposite reaction force $(iv)$ on the horse in the forward direction. The cart moves forward because the forward reaction force $(iv)$ is greater than the backward frictional force $(v)$ acting on the cart. Thus,the system moves due to the net unbalanced force in the forward direction.