In which of the following scenarios is the person reinforced on a variable interval schedule?

In hopes of winning a big jackpot, Patricia plays the slot machines at a nearby casino.

Shmuel earns extra money by shoveling his neighbor's driveways and sidewalks in the winter. He charges $$20$ per job but only shovels for snowfalls of $1$ inch or more.

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.

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.