D. it has been improved by having unwanted substances removed
Answer:
Fixed-interval schedule
Explanation:
A Fixed-interval schedule is a schedule of reinforcement where the first response is rewarded only after a specified amount of time has elapsed. This schedule causes high amounts of responding near the end of the interval but much slower responding immediately after the delivery of the reinforce.
A Fixed Interval Schedule provides a reward at consistent times.
This is called tariffs. I read that somewhere in my history class.
