Every time Joanna begins to bite her hand, her mother gives her a stress ball to use as an alternative. It may be that observing mom hold the stress ball causes Joanna to bite her hand, or that biting her hand causes her mother to keep a stress ball nearby. This is an example of which method by Mill?

a. Method of difference
b. Joint method of agreement and difference
c. Method of concomitant variation
d. Method of agreement