Which of the following statements about economic methodology is correct?

a.	An economic law is a cause-and-effect relationship that has been verified by a controlled experiment.
b.	The test of a theory's validity is its usefulness in explaining reality.
c.	An economic law is a statement about group behavior that is based on the observed behavior of a single individual.
d.	An economic law, because it is based on probabilities, can only be expected to be true about half the time.