Answer: B. Salt and rust are corrosives that have damaging effects on a city’s infrastructure.
Explanation: in a text, a main idea is important information that tells more about the overall idea of a paragraph or section of a text. In the given excerpt from "The City Without Us” by Alan Weisman we can see the description of how the salt used on the roadways on winter, is very corrosive because it eats steel, and it also says that with no people, there won't be salt, but there will be rust which is also corrosive. According to this information, the main idea of the paragraph is best represented by option B: Salt and rust are corrosives that have damaging effects on a city’s infrastructure.
