Which of the Following Is an Example of a Fixed-Interval

Which of the following is an example of a fixed-interval schedule of intermittent reinforcement for recycling?

A) Receiving a set discount on your monthly services bill after signing up for the city recycling program.
B) Receiving occasional coupons from the city after signing up for the recycling program.
C) Receiving $1 off your monthly service bill for every pound of material you recycled that month.
D) Receiving occasional coupons from the city as long as you put something in your recycling bin each month