Conversation so far:
name the one major city in texas that has no zoning ordinances
Houston is the only major city in the U.S. without zoning ordinances.
what else is special about Houston besides having no zoning ordinances
Houston is the most populous city in the U.S. state of Texas.
what is the total area of the city Houston
Houston has a total area of 637.4 square miles.

Question: what is the basis of the city's economy
Self-contained rewrite: what is the basis of Houston's economy