Employees at Smith's Club, a discount grocery chain, know that they have to stay on their toes at all times. Mr. Smith, the company founder, has been known to put on disguises and make unannounced visits to stores to make sure the staff is treating customers politely. This is an example of:________

a. a variable ratio schedule.
b. continuous reinforcement.
c. a variable interval schedule.
d. a fixed interval schedule.
e. a fixed ratio schedule.