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The Interstate Bridge over the Apache River, built in the 1950s, shows a substantial amount of rust: as much as 45% of its surface is coated in rust. Community activists have argued that the bridge presents a hazard: it is likely to collapse in places where it has rusted through. Professors of mechanical engineering at the local university did an extensive analysis of the bridge. These professors and their graduate students determined that 98% of the rust on the bridge exists on the surface only, and actually seals the underlying steel from the corrosive influence of the elements. The results of this academic study suggest that the bridge is safe for continued use.

In the argument given, the two portions in boldface play which of the following roles?


The Interstate Bridge

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