A metal block is sliding along an infinite, frictionless, horizontal plane. There are only two forces acting on the metal block: gravity and an upward force from the plane. Which of the following best describes the metal block's motion?

The correct answer is C. The metal block will move at the same speed forever.

Since the plane is frictionless, there is no frictional force to slow the block down. The only forces acting on the block are gravity (acting downward) and the upward normal force from the plane, which balances the gravitational force. Therefore, there is no net force acting on the block in the horizontal direction, and it will continue to move at a constant speed indefinitely, according to Newton's first law of motion.