Josiah Checks His Electronic Mail Several Times Throughout the Day

Multiple Choice

Josiah checks his electronic mail several times throughout the day. Some days there is mail each time he checks; sometimes several days go by with no new messages arriving. In this example, Josiah's behaviour of checking his electronic mail is being reinforced on

A) a variable-interval schedule.
B) a fixed-ratio schedule.
C) a fixed-interval schedule.
D) a variable-ratio schedule.