The following is an example of a variable interval schedule of reinforcement: A. For every 60 minutes that Jacob practices the violin, he earns $1 from his parents.
B. For every 3 songs Jacob practices on his violin, he earns $1 from his parents.
C. Jacob's parents place a dollar bill in his piggy bank when he practices his violin for an average of 8 songs.
D. Jacob's parents place a dollar bill in his piggy bank when he plays his violin for an average of 60 minutes each day.