main idea. A. Peasants in southern Europe developed a three-field system of crop rotation that was based on agricultural practices used by Roman farmers. B. By rotating their land, using three fields rather than the traditional two, many northern farmers were able to harvest twice during the year, significantly increasing their annual production. C. Agricultural innovations led to population growth everywhere in Europe, and the resulting towns were the reason that trade increased. D. By using wheeled plows and replacing horses with oxen, farmers could plow the soil of Northern Europe more quickly, contributing to economic improvements. E. As European town markets expanded to become trade centers selling goods from distant locations, the social and economic power of merchants increased. F. A growing system of trade that included long-distance merchants and large annual markets contributed to innovations in economic practice and the flourishing of town culture.