On Monday,Johnny's Mother Gave Him Cookies and Milk After He

On Monday,Johnny's mother gave him cookies and milk after he had played quietly for 10 minutes.On Tuesday,she required 20 minutes of quiet play before treat time.On Wednesday,she gave him cookies only after he played quietly for a full half hour.Johnny was taught to play quietly for extended periods through

A) secondary reinforcement.
B) spontaneous recovery.
C) shaping.
D) fixed-ratio scheduling.