Read this excerpt 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.
Which sentence best states the main idea of this section of the chapter?
A. The ailanthus is the most common landscape tree planted in urban America.
B. Ailanthus trees need very little soil and nurturing to grow and thrive.
C. Ailanthus trees are nearly impossible to eradicate once they gain a foothold.
D. Ailanthus trees don't need sunlight, as they can grow in subway tunnels.