I Am Trying to Start My Lawn Mower

Multiple Choice

I am trying to start my lawn mower.I pull the starting cord 20 times and it doesn't start.I then wait 1 min and pull the cord another 20 times and it still doesn't start.I then decide to take a break.After waiting for 5 min,I come back and pull the cord and the lawn mower starts.This is an example of which type of schedule of reinforcement?

A) variable-interval
B) variable-ratio
C) DRO schedule
D) DRL schedule