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 behavior 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.