Which of the following is an example of a fixed ratio schedule of partial reinforcement? a. Hoping to catch a message from her boyfriend, Wendy checks her e-mail again and again throughout the day. b. Jaime has a paper due once a month in his sociology course. His productivity is low at the beginning of the month and high as the deadline approaches. c. Sam earns extra money by driving a shuttle bus from the airport to the hotel. He is paid $20 per trip, but Uber drivers and taxis sometimes pick up all the passengers going to the same hotel and Sam has to wait for the next flight to arrive. d. In hopes of winning a big jackpot, Penny plays the slot machines at a nearby casino.