Which would best explain the decline of an early civilization? a. a river and its tributaries change course over time. b. people settle near a river where it empties into the sea. c. the natural features in an area include high mountains, large deserts, and oceans. d. a large river floods annually.

The correct answer is D) a large river floods annually.

The statement that best explains the decline of an early civilization would be a large river floods annually.

We are talking ancient civilizations such as Sumeria, the oldest civilization known to man. The Sumerians settled in the middle of the Euphrates and Tigris Rivers, in the Middle East region, modern-day Irak. Another example could be the Egyptians that settled in next to the banks of the Nile River in Africa, or the Indus Valley civilization that settled in next to the Indus River.