Mrs. French is implementing a system to encourage "on-task" behavior when her students are doing seat work. She has a timer that "beeps" every five minutes. If, each time the timer beeps, all the students are conscientiously working, the class is given free time to play games and work at the computer the last half hour of the day. Of the following, the best description of Mrs. French's technique would be called: 

a. continuous reinforcement. 

b. fixed-interval reinforcement. 

c. variable-interval reinforcement. 

d. variable-ratio reinforcement.