A recent study confirms that nutritious breakfasts make workers more productive. For one month, workers at Plant A received free nutritious breakfasts every day before work, while workers in Plant B did not. The productivity of Plant A' s workers increased, while that of Plant B' s workers did not. Which one of the following, if true, most strengthens the argument? ['Workers from Plant A took fewer vacation days per capita during the month than did workers from Plant B.', 'Few workers in Plant B consumed nutritious breakfasts during the month of the study.', 'Workers in Plant B were more productive during the month of the study than were workers from Plant A.', 'During the month before the study, workers at Plant A and Plant B were equally productive.']