Question 976069
c has increased, and c is the variable.  The constant is the only thing that doesn't change, didn't change, and has to be the locomotive.

If c increased, the length of the passenger cars increased, or D.

If the locomotive were 12 feet, the function would be 9c +12 
If the locomotive were 9 feet, the function would be 10c
If the passenger cars were 9 feet and the locomotive stayed the same, it would be the original function.