12.	Identify the type of sampling (stratified, systematic, convenience, or cluster) that best describes each of the following cases:
	a.	The U.S. Department of Corrections collects data about returning prisoners by randomly selecting five federal prisons and surveying all of the prisoners in each of the prisons.
	b.	A college teacher surveyed all of her students to obtain sample data consisting of the number of credit cards students possess.
	c.	A man was an observer at a town's sobriety checkpoint at which every fifth driver was stopped and interviewed.
	d.	In a study of college programs, 567 students were randomly selected from those majoring in mathematics, 1236 students were randomly selected from those majoring in business and 822 students were randomly selected from those majoring in psychology.