Although southern Mesopotamia is a hot, desert region, why was it able to become the cradle of civilizations?

Question

A. Nomads developed aggressive herding techniques
B. Flooding of the Tigris and Euphrates river brought good soil
C. Farmers developed technology for irrigation***
D. Tribes built deep wells and extensive cooling structures for good crops***
E. Engineers developed ways of extracting usable fresh water from the salt water of the Persian Golf

I think it might be B or D too, can you help me? Thx!

Answer 1

C is right; D is not.