A hockey puck slides along the ice. There is a frictional force between the ice and puck, and this is the only horizontal force on the puck. Which of the following must be true?

A. The puck will slow down and eventually come to rest.
B. The puck will continue to move at constant velocity.
C. The puck will speed up.
D. The frictional force points in a direction opposite that of the puck’s motion.
E. Both (a) and (d) are true

To determine the correct answer, let's analyze the situation. We know that there is a frictional force between the ice and the puck, and this is the only horizontal force acting on the puck.

Now, let's consider Newton's first law of motion, also known as the law of inertia. It states that an object at rest will stay at rest, and an object in motion will remain in motion with a constant velocity, unless acted upon by an external force.

In this scenario, since there is a frictional force acting on the puck, we can conclude that it is being subjected to an external force. Therefore, the puck will not continue to move at a constant velocity (B) because the frictional force opposes the puck's motion.

Furthermore, the frictional force always acts in the direction opposite to the motion of the object. This is true in this particular scenario as well. The frictional force is directed opposite to the motion of the puck. Therefore, statement (D) is true.

Now, let's consider statement (A). Since there is a frictional force acting on the puck, it will cause the puck to slow down. As the velocity decreases, the frictional force will eventually bring the puck to a stop. Therefore, statement (A) is also true.

By analyzing the scenario and the laws of motion, we can conclude that both statements (A) and (D) are true. Therefore, the correct answer is option (E), "Both (a) and (d) are true."