C. when ducal states Yan, Zhao, Wei, and Qin were frequently plundered by the nomadic peoples living north of the Yinshan and Yanshan mountain ranges.

• 后来在221年时，秦征服了其他国家和统一中国，秦始皇下令连接这些个人墙壁和进一步扩展的基础，目前长城。