Threats from nomadic people in the north were 【C1】______ throughout Chinese history. They were continually attacking the Chinese northern 【C2】______ . With each new emperor, came endless debate about how to 【C3】______ the barbarians. There were four options. 【C4】______ offensive campaigns to drive them away or to destroy them create defensive garrisons develop diplomatic and 【C5】______ ties with them, or build a wall to keep them out. All the options were 【C6】______ at various times. Experience showed that 【C7】______ campaigns were too costly and very risky, 【C8】______ defensive garrisons could not respond quickly enough 【C9】______ lightning attacks along a long border. The third option would seem to be a very 【C10】______ one and it was in fact tried successfully 【C11】______ a couple of occasions. Wall building became the most favored option in many dynasties. The three dynasties which 【C12】______ the most walls were the Qin, the Han and the Ming. The effectiveness of the Great Wall in history is still a controversial 【C13】______ . Historical records show that the wall 【C14】______ at many times 【C15】______ repel invaders. Only on two occasions when a dynasty weakened from 【C16】______ were invaders from the north 【C17】______ advance and conquer. 【C18】______ , scholars think the Chinese wall builders were themselves responsible for the unrest on the border. The nomads were people who did not farm, so they 【C19】______ trade with China for many essentials. When the Chinese refused to trade with them, they had no option 【C20】______ raid border towns. 【C1】