A horse is pulling a sled. If the force exerted by the horse on the sled has the same magnitude as but the opposite direction to the force exerted by the sled on the horse, how are the horse and the sled able to start moving forward? Choose all the explanations that you consider to be correct.
a. If the system is the horse and the sled, then to explain the system's motion we need to consider the external forces exerted on it. The sum of these forces causes the forward acceleration of the system.
b. If we consider the horse to be the system, and if the sum of the friction force and the force exerted by the sled points in the forward direction, the horse will accelerate in the same direction.
c. For a brief instant right before the sled starts moving, the horse pulls slightly harder on the sled than the sled pulls back on it. This slight imbalance allows them both to start moving.
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