Katrina Is Trying to Put a Coin into a Vending

Multiple Choice

Katrina is trying to put a coin into a vending machine in her office.Sometimes the machine will take a coin on the first try,and other times it can take up to five or six tries before the coin is finally accepted.On which type of schedule is inserting a coin into the vending machine reinforced in this example

A)  intermittent 
B)  short-delay 
C)  continuous 
D)  noncontingent