What is the imaginary line dividing two watersheds called

The imaginary line that connects those high points is called the watershed divide. Precipitation that falls inside the watershed divide will flow down to the water body (like a river or lake) at the lowest point in that landscape. The watershed is usually named after that water body.

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.