单项选择题

Threats from nomadic people in the north were (31) throughout Chinese history. They were continually attacking the Chinese northern (32) . With each new emperor, came endless debate about how to (33) the barbarians. There were four options. (34) offensive campaigns to drive them away or to destroy them; create defensive garrisons; develop diplomatic and (35) ties with them, or build a wall to keep them out.
All the options were (36) at various times. Experience showed that (37) campaigns were too costly and very risky, (38) defensive garrisons could not respond quickly enough (39) lightning attacks along a long border. The third option would seem to be a very (40) one and it was in fact tried successfully (41) a couple of occasions. Wall building became the most favored option in many dynasties. The three dynasties which (42) the most walls were the Qin, the Han and the Ming.
The effectiveness of the Great Wall in history is still a controversial (43) . Historical records show that the wall (44) at many times (45) repel invaders. Only on two occasions when a dynasty weakened from (46) were invaders from the north (47) advance and conquer. (48) , 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 (49) trade with China for many essentials. When the Chinese refused to trade with them, they had no option (50) raid border towns.