Final answer:
Sumer is classified as a civilization because of its complex social structures and advanced agricultural techniques. One great invention of the Sumerians was writing, specifically cuneiform, which revolutionized record-keeping. The wheel was also an important invention of the Sumerians.
Step-by-step explanation:
Historians classify Sumer as a civilization because it met certain criteria that define a civilization. Sumer had complex social structures, including a ruling class, priests, and merchants. It also had advanced agricultural techniques, such as irrigation systems, which allowed for surplus food production. The Sumerians developed a writing system called cuneiform, which was one of the greatest inventions of the Sumerians. Cuneiform enabled them to record laws, religious tracts, and property transactions. Another significant invention of the Sumerians was the wheel, which revolutionized transportation and trade.