Josiah Checks His Text Messages Quite Often Throughout the Day

Multiple Choice

Josiah checks his text messages quite often throughout the day.Some days there is a text each time he checks; sometimes several hours go by with no new messages arriving.How is Josiah's behaviour of checking his messages 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