Which among the following describes the law of inertia?

A. “A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon an external force”

B. For every action, there is an equal and opposite reaction.
C. "The force that resists motion whenever two materials or media are in contact with each other, and are sliding or rolling over each other."
D. The force acting on an object is equal to the mass of that object times its acceleration”