Answer:
C) river valleys and D) fertile plateaus
Step-by-step explanation:
As some people may know, Mesopotamia is a region in the Fertile Crescent that resides between the Tigris and Euphrates Rivers. Both rivers had flooded regularly, which led to fertile land and the civilization that we know today as Sumer. Over time, regular flooding of the Tigris and Euphrates led to contamination with salt that also led to plant death and the eventual collapse of Sumer.
Mesopotamia was located in modern-day Iraq and was known for being arid, but flourished from the floods. Therefore, thick forests is not a correct answer. As for abundant rainfall, it wasn't always plentiful, but the reason why Sumer was as successful as it was is the frequent flooding of the Tigris and Euphrates Rivers. As for river valleys, it's pretty obvious that this is right. Lastly, Mesopotamia at the height of Sumer was very fertile, and it was a plateau.
Therefore, the answers are C) river valleys and D) fertile plateaus.