Answer:
C. planets; moons
Explanation:
From Newton's law of inertia, a body keeps moving in straight line and or be at rest, until an external unbalanced force acts on it.
The planets remain in the orbits around the sun and similarly moons remain in the orbits around their planets due to gravity.
Sun's gravitational pull keeps the planets in their orbits, just as a planet's gravitational pull keeps its moons in their orbits.
In the absence of gravity, the planets and moons would move in straight line path.
