The sentence that best states the main idea of this section of the chapter is:
B. Salt and rust are corrosives that have damaging effects on the city's infrastructure.
The paragraph describes the salt on the roadways as enemy number one. This salts eats up the steel and creates a dreadful job once it is combined with antifreeze, oil and snowmelt. If there were no people in the city, the city wouldn't have salt but there would be rust because no one would paint it. In that excerpt the author presents how salt and rust are problematic, and destroy the city's infrastructure.