The ancient Romans understood the principles of water power very well, and in some outlying parts of their empire they made extensive and excellent use of water as an energy source. This makes it all the more striking that the Romans made do without water power in regions dominated by large cities. Which one of the following, if true, contributes most to an explanation of the difference described above in the Romans' use of water power? ['In heavily populated areas the introduction of water power would have been certain to cause social unrest by depriving large numbers of people of their livelihood.', 'In the areas in which water power was not used, water flow in rivers and streams was substantial throughout the year but nevertheless exhibited some seasonal variation.', 'In most areas to which the use of water power was not extended, other, more traditional sources of energy continued to be used.', 'The ancient Romans were adept at constructing and maintaining aqueducts that could carry quantities of water sufficient to supply large cities over considerable distances.']