Which statement explains how planets move in orbit as supported by Newton’s first law of motion?

Newton’s first law says an object in motion keeps moving at a constant velocity unless an outside force acts on it. A planet’s motion would otherwise continue in a straight line; gravity acts as an outside force that continually changes its direction and keeps it in orbit. So choice C, that planets in motion have a constant speed unless acted on by an outside force, is the best match.

AI-written and checked against the verified answer.