A consultant wants to ask workers at a factory about the workers' job satisfaction. Which of the following best describes a stratified sample of workers?
a. The consultant forms groups of workers based on the workers' shifts. Then, he selects workers at random from each group.
b. All of the workers on the second floor of the factory building are easily accessible. So, the consultant selects the first workers from the second floor to arrive at work on a particular day.
c. The consultant forms groups of workers based on the lengths of time the workers have worked at the factory. Then, he randomly chooses groups and selects all of the workers in these groups.