According to Newton's first law, what does the law describe?

• A. An object remains at rest or moves with constant velocity unless acted on by a net external force.
• B. The acceleration of an object is always opposite in direction to the net force acting on it.
• C. For every action force there is an equal and opposite reaction force.
• D. The net force on an object is equal to its mass times its acceleration, $F=ma$.

Answer: (A)