D
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
A student is standing on a skateboard that is not moving. The total mass of the student and the skateboard is 50 kilograms. The student throws a ball with a mass of 2 kilograms forward at 5 m/s. Assuming the skateboard wheels are frictionless, how will the student and the skateboard move?

### Choices:
A. forward at 0.4 m/s
B. forward at 5 m/s
C. backward at 0.2 m/s
D. backward at 5 m/s

### Answer:
