Why Do Variable-Ratio Schedules Produce Steady Rates of Responding

Why do variable-ratio schedules produce steady rates of responding?

A) Reinforcers are delivered on a regular basis.
B) Aversive consequences are less likely to occur with this schedule of reinforcement.
C) It is not possible to predict which response will lead to the delivery of a reinforcer.
D) The timing between reinforcers is predictable although the number of responses may vary.