Which of the following is the topic sentence?
The greatest gift the Sumerians gave the world was the invention of writing. The Sumerians were a wealthy people. They needed some way to keep track of what they owned. They began drawing pictures. They used a reed as a pen. They drew on soft pieces of clay. The soft clay was then dried in the sun. The tablet became a permanent record. Later, the Sumerian drawings changed into wedge-shaped symbols. This kind of writing is called cuneiform. By putting symbols together, the Sumerians could write entire sentences.