Which of the following is an example of a fixed ratio schedule of partial reinforcement?

Which of the following is an example of a fixed ratio schedule of partial reinforcement?
 
  a. In hopes of winning a big jackpot, Penny plays the slot machines at a nearby casino.
  b. Sam earns extra money by mowing his neighbors lawns in the summer and shoveling their driveways and sidewalks in the winter. He charges 20 per job.
  c. Hoping to catch a message from her boyfriend, Wendy checks her e-mail again and again throughout the day.
  d. 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.