A Good Example of a Variable Ratio Schedule Is

A good example of a variable ratio schedule is

A) a mother who checks her son's room every fifteen or twenty minutes to make sure he is cleaning it.
B) a gambler who plays a slot machine for hours after winning twenty dollars.
C) a teacher who checks her students' progress after they have completed five problems each.
D) a family that sits down to dinner every day at 5:30, without exception.