A)  If an object’s acceleration is constant, then it must move in a straight line.

 B)  If an object’s acceleration is zero, then it’s speed must remain constant. 

 C)  If an object’s speed remains constant, then its acceleration must be zero. 

 D)  If the object’s direction of motion is changing, then its acceleration is not zero. 

 A)  If an object’s acceleration is constant, then it must move in a straight line.

 B)  If an object’s acceleration is zero, then it’s speed must remain constant. 

 C)  If an object’s speed remains constant, then its acceleration must be zero. 

 D)  If the object’s direction of motion is changing, then its acceleration is not zero. 

In parabolic motion, a projectile experiences only the constant direction due to gravity but the velocity does not point in the same direction. Eliminate (A). Zero acceleration means no change in speed (or direction), making (B) and (D) true. An object whose speed remains constant but whose velocity vector is changing direction is accelerating. Eliminate (C).