During the fall of empires like China, Rome and India, they had nomadic invasions during a time of weakness in their imperial systems.
The last option is the correct answer to this question. These empires became exposed to enemies from all sides.
The enemies took the weakness that they saw in these empires as an opportunity to wreck havoc and also get some areas of interests from them.
The constant invasion that the Chinese people suffered was what led to the building of the great walls of China by the state. In other to keep off the invaders.
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