The correct answers are: renew - fertile - silt.
The floods of the big rivers were crucial for the existence and prosper of the river civilizations. This was due to the repetitive natural cycle of the rivers, when they flooded the surrounding area and they were renewing the fertile silt every year. The renewing of the fertile silt meant that the soil was of high quality every year, and because of that people were able to produce large amounts of food, not just for their own needs, but also for trade.
