Answer is B.
If using a chart the points would be
1 2 3 4 5 6 7 8 9 10
5 10 15 20 25 30 35 40 45 50
Graph B matches these points perfectly as the Y values increase by 5 and the X values increase by 1. This is where the term per hour comes into play. Its 5 miles every 1 hour.