The Woye south of the Yinshan Mountain is not only a garden for the nomads, but also a springboard for them to enter the Central Plains. As long as they occupy the Woye, they will cross the Yellow River and enter the Fen or Yellow River valleys. If they lose this Woye, Lost the basis for survival, history contains "After the Xiongnu lost Yinshan, never crying" is the reason; on the other hand, if the Han people want to exclude the threat of nomads from the northwest, they must also guard YinshanӸڬ otherwise these riding nations will cross the Ordos desert and enter the heart of the Han residential area. What is the main point of this passage? ['The area south of Yinshan is a fertile field.', 'The fertile ground south of Yinshan is the basis for the survival of nomads.', 'The Woye south of Yinshan is not only a garden for the nomads, but also a springboard for them to enter the Central Plains.', 'The Woye south of Yinshan is extremely important to nomads and Hans.']