C. by digging deeper into the tels of southern Mesopotamia.
"Tel" is an Arabic word for a tall hill or mound. These mounds are created by people building and rebuilding on the same city site for hundreds of years or more.
One of the most ancient settlements in Mesopotamia was Eridu. The tell at Eridu is a very large mound, hundreds of meters across. Settlement there dates back over 5,000 years BC. The height of the tel is over 20 feet, and includes ruins of houses, temples, and cemeteries built on top of one another for several thousand years..