Answer:
B. One number line increases by 9, and the other increases by 6.
If the team scores an average of 9 goals per season, then in 6 seasons, they would score an average of 6 times 9 goals, which is 54 goals. On the double number line, one scale could represent the number of seasons (increasing by 6) and the other scale could represent the number of goals scored (increasing by 9). Therefore, one number line increases by 6, and the other increases by 9.