Which of the Following Is an Example of a Variable

Multiple Choice

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 shoveling his neighbor's driveways and sidewalks in the winter.He charges $20 per job but only shovels for snow falls of 1 inch or more.
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.