The higher a dam, the more exposed it is to forces that can cause it to collapse. Of the world' s hundreds of arch dams, more than half are over 100 meters high. Yet all six of the arch dam collapses that have occurred during the past 40 years have occurred in arch dams under 100 meters high. Which one of the following, if true, most helps to resolve the apparent paradox? ['The larger the structure, the more careful the attention it received in design and construction.', 'The higher a dam must be, the greater the likelihood that it will be built as an arch dam.', 'Arch dams are generally more complicated and more costly to construct than other types of dams of comparable size.', 'Arch dams under 100 meters high are less subject to collapse than are dams of other types also under 100 meters high.']