Question

Objects in space that are moving at a constant velocity in a straight line ___________.

Answer 1

The best answer is A) keep moving at a constant velocity until some forces act on them

As the man you're probably tired of hearing about said:

"Every object persists in its state of rest or in uniform motion in a straight line unless a new force acts upon it" 
This is Isaac Newton's 1st law of motion, or the law of inertia. 

Put more simply, objects in motion tend to stay in motion, and tend the maintain the same velocity (direction and speed) and objects at rest tend to stay at rest. 

Answer 2

A. Keep moving at a constant velocity until some forces act on them