求翻译：17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？

17 medium to mid-18th century westward movement in addition to the brutal heartless massacre, a sense of the story of heaven and earth, there is a difficult exploration and silent work. This period agriculture also Westward Movement of the extensive and profound changes in the development and growth

17 mid-term to the 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 profound change, th

17 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 changes, thes