The correct choice would depend on the context provided regarding the forces acting on the nail. However, if we are considering a scenario where a nail is being hammered into a surface (for example, wood), we can analyze the forces:
- The hammer applies a downward force on the nail.
- According to Newton's Third Law, the nail exerts an equal and opposite force on the hammer.
- If there are no other forces acting on the nail in the upward direction (like resistance from the wood), we can conclude that the downward force from the hammer will cause the nail to accelerate downward.
Thus, in the context of the nail being hammered in, the appropriate response would be:
a. accelerate downward because there are unbalanced forces.
However, if there is a situation where the nail is at rest and all forces acting on it are balanced (such as a nail lying flat on a surface with equal forces acting in opposite directions), then it would indeed remain at rest. But generally, in the context of hammering a nail, the first option is the correct one.