Josiah Checks His Email Several Times Throughout the Day

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

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