Read this passage from "The City Without Us" by Alan Weisman: The weeds are followed by the city's most prolific exotic
species, the Chinese ailanthus tree. Even with 8 million
people around, ailanthus-otherwise innocently known as
the tree-of-heaven-are implacable invaders capable of
rooting in tiny chinks in subway tunnels, unnoticed until
their spreading leaf canopies start poking from sidewalk
grates. With no one to yank their seedlings, within five
years powerful ailanthus roots are heaving up sidewalks
and wreaking havoc in sewers.
Which statement best explains how irony is used in the passage?