You Are Watching a Rat Press a Bar in a Skinner

You are watching a rat press a bar in a Skinner box. As you look at the cumulative record, you notice that there are significant pauses in responding after reinforcement is delivered. Moreover, the pauses are not flat, they are curved or scalloped. What kind of schedule is the rat being trained on?

A) continuous reinforcement
B) fixed ratio
C) fixed interval
D) variable interval
E) variable ratio