An object travels down a ramp at a constant acceleration. The object experiences a force of friction and a gravitational force. Which of the following could be true about the motion of the object?

a. The force of friction acts in the same direction as the object travels.
b. The force of friction between the surface and the object is less than the component of the gravitational force that is parallel to the ramp.
c. If the object increased in mass, the object's acceleration would change.
d. If the object increased in mass, the normal force exerted on the object would remain the same.