What was a major cause of the
decline of empires in ancient times?

Invasions by Barbarian tribes. The most straightforward theory for Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire's borders.

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explanation:

#2 the answer is d

#3 the answer is a

#4 the answer is b