MrHall Tells His Students That They Can Do Whatever They

Mr.Hall tells his students that they can do whatever they want for the first ten minutes of class, but then must turn their attention to the day's assignment.The students are delighted with their ten minutes of "free time" but then do not attend to the assignment when it is time for them to do so.From an operant conditioning perspective, what mistake has Mr.Hall made?

A) The reinforcement was not immediate.
B) He has used negative reinforcement instead of positive reinforcement.
C) Free time is not an effective reinforcer for most students.
D) The "reinforcer" is not contingent on the response.