One year ago, a municipality banned dishwasher detergents containing phosphates. Anecdotal evidence indicates that many residents continued to use detergents containing phosphates; they just purchased them from out-of-town stores. However, it is clear that some residents did switch to phosphate-free detergents, since phosphate pollution from the municipal wastewater treatment plant decreased significantly in the past year. The answer to which one of the following questions would most help in evaluating the argument above? ['Did municipal officials try to stop people from bringing detergents containing phosphates into the municipality?', "Were any changes made in the past year to the way the municipality's wastewater treatment plant treats phosphates?", 'What pollutants, if any, are present in phosphate-free dishwashing detergents?', 'Why did many residents continue to use detergents containing phosphates?']