You're riding on a train that's moving at 50 km/h, and you roll a ball down the aisle at 2 km/h in the same direction as the train is moving. If a person standing on the ground near the train track could see the ball, how fast would it appear to be moving?
A. 2 km/h
B. 48 km/h
C. 50 km/h
D. 52 km/h