C or d would be the answer
The development of irrigation systems made farming easier, as water was more easily accessible to crops. This meant that food could be produced more efficiently. More efficient farming techniques meant that food was more plentiful, where people weren't constantly scrambling for their next meal or going days without eating. It also gave people the ability to specialize in certain trades, creating better quality of goods and advancing technology in civilizations.
By their shared language,culture,and climate.
