Which of the following is an example of a variable interval schedule of partial reinforcement?

Which of the following is an example of a variable interval 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. Marcus takes his daughter Rebekah fishing, and explains that you never know how long you have to leave your line in the water before a fish will bite.
  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.