Read this excerpt from “the city without us” by alan weisman:

enemy number one is the salt that highway departments spread on the roadways each winter -ravenous stuff that keeps eating steel once it’s done with the ice. oil, antifreeze, and snowmelt dripping from cars wash salt into catch basins and crevices where maintenance crews must find and flush it. with no more people, there won’t be salt. there will, however, be rust, and quite a bit of it, when no one is painting the bridges.

which sentence best states the main idea of this section of the chapter?
a. salt flushing and painting rusty bridges consume a large part of a city’s budget.
b. salt and rust are corrosives that have damaging effects on a city’s infrastructure.
c. salt is the most damaging compound that people can use to fight snow and ice.
d. salt can be used as a metaphor for a monster that has a voracious appetite.