Joe owned a small repair shop. Each day, he would check the mail to see if any of his customers mailed in a payment for the work he had done for them. Some days, he would receive a check or two

At other times, he would have to wait days before getting another payment. What schedule of reinforcement is evident here?5.13 5.12 5.11 5.10 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 5.1
a. fixed interval
b. fixed ratio
c. variable interval
d. variable ratio

c