Which of the following is the correct definition for a fixed interval schedule?
a. It is a rule specifying the number of correct responses a subject must give to receive reinforcement.
b. It is a rule for delivering reinforcement after varying numbers of correct responses.
c. It is a rule for how long a set time period must pass before a subject's correct response can be reinforced.
d. It is a rule for how long a varying time period must pass before a subject's correct response can be reinforced.