Grandpa joe loves to fish. while fishing, joe puts his line in the water and waits there until he feels the line tug. on what reinforcement schedule is grandpa joe rewarded?

The correct answer is variable interval.
Variable interval means that a particular reinforcement is given after some time has passed. In the case above, grandpa Joe wants to catch a fish, which is his reinforcement. However, he cannot catch it immediately - he has to put his line in the water first, and then wait for some time to feel the line tug. Only then can he get his reinforcement due to variable interval schedule. 

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