TOEFL Listening: ETS-TOEFL听力机经 - 28257M181YSGORAJP$

Why does the professor mention the Sumerians? A. To imply that the markings on the cave walls in France are consistent with early writing system B. To point out that the Sumerians developed their writing system on cave walls C. To help students understand the symbolic meanings of the large paintings of animals D. To explain why the system of signs on the cave walls in France did not change over time