Answer:
d. irrigated crops
Step-by-step explanation:
In subtropical areas, irrigation led to highly developed agriculture and made it possible to achieve a yield sufficient to support a high population.
Rivers overflowing silt provided water for irrigation and enriched the soil.
Fields with irrigation and flood control require hydraulic structures in order to bring water where it is needed, when it is needed.
Technology was developed to drain swamps, build dams, levees, canals, terraces, and all this must be cleaned and maintained.