Harrison sends an e-mail to his boss. Harrison expects a response within five minutes, but he can’t help himself and checks every 30 seconds. After five minutes, a replying e-mail is delivered just as he expected. Which of the following statements is true? A. Harrison is on a fixed-interval schedule of reinforcement. B. Harrison is on a variable-interval schedule of reinforcement. C. Harrison is on a fixed-ratio schedule of reinforcement. D. Harrison is on a variable-ratio schedule of reinforcement.