Although water in deep aquifers does not contain disease-causing bacteria, when public water supplies are drawn from deep aquifers, chlorine is often added to the water as a disinfectant because contamination can occur as a result of flaws in pipes or storage tanks. Of 50 municipalities that all pumped water from the same deep aquifer, 30 chlorinated their water and 20 did not. The water in all of the municipalities met the regional government' s standards for cleanliness, yet the water supplied by the 20 municipalities that did not chlorinate had less bacterial contamination than the water supplied by the municipalities that added chlorine. Which one of the following can properly be concluded from the information given above? ["The regional government's standards allow some bacteria in municipal water supplies.", 'Chlorine is the least effective disinfecting agent.', 'Where accessible, deep aquifers are the best choice as a source for a municipal water supply.', "A municipality's initial decision whether or not to use chlorine is based on the amount of bacterial contamination in the water source."]