Answer:
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.
