Which of the following must always be true?

 (I) If an object’s acceleration is constant, then it must move in a straight line. (II) If an object’s acceleration is zero, then its speed must remain constant. (III) If an object’s speed remains constant, then its acceleration must be zero.

 (A) I and II only
 (B) I and III only
 (C) II only
 (D) III only
 (E) II and III only