It is not A or C, and D isn't specific enough (over what amount of time? is it flying up? down? It is going forward at the same height?) The most likely thing to stay consistent is the cost of fabric and it's length.
Say 1 foot of fabric is $3 (assuming there's no discount for additional feet, but math isn't that consistent with actual store sales), then 2 is $6, and 3 is $9. That is a constant rate of change.
