An asteroid leaves our solar system and continues moving in the same direction at the same speed for millions of years. What allows the asteroid to keep moving?

A. Acceleration

B. Friction

C. Inertia

D. Force

'Inertia' is the property of all matter by which it tends to remain in constant uniform motion unless acted upon by an external force. (C)

C. Inertia

Explanation:

Inertia is the tendency of an object to maintain its state, either motion or rest. A moving object will keep on moving and an object which remains at rest will continue to remain at rest, as long as no other force from outside acts on it.

So the asteroid will continue to move as long as no other force acts on it. This inertia of motion is the one which keeps the asteroid moving. It can accelerate only if some force from outside will try to push it. Since it does not happen in space, it is inertia that maintains its motion.