求翻译：17世纪中期到18世纪中期美国的西进运动中，除了残暴不仁的屠杀，感天动地的故事，还有着艰难的探索与默默的耕耘。而此时期的农业也在西进运动中产生了广泛而深刻的变革，这些变革对美国的发展和壮大有着直接的关系。美国西进运动中农业的变化主要体现在三个方面：1、生产要素发生变化，主要集中在劳动力劳动工具和劳动对象。2、生产方式发生变化。3、国家政策出现变动。是什么意思？

The mid-17th century to the mid-18th century the U.S. Westward Movement, in addition to the brutal and unscrupulous massacre, the sense of days of moving to the story, there is a difficult exploration and silent work. This period agriculture also Westward Movement of the extensive and profound chang

Middle of the 17th century to mid 18th century movement in the United States in the west, with the exception of the brutal killings, heartless and sense of the day, there is also a difficult story of exploration and the silence. And this period is also the agriculture in the West, the broad and prof

17th century medium-term to the 18th century, the medium-term United States westward movement, in addition to the cruel heartless slaughter, stories of the day, there is hard exploration and hard work in silence. Agriculture is also produced in the westward movement of the extensive and profound cha