Which of the following statements are true?
(a) an object can move even when no force acts on it.

(b) if an object isn't moving, no external forces act on it.

(c) if a single force acts on an object, the object accelerates.

(d) if an object accelerates, a force is acting on it.

(e) if an object isn't accelerating, no external force is acting on it.

(f) if the net force acting on an object is in the positive x-direction, the object moves only in the positive x-direction.