Empires can fall for a variety of reasons, and while all the given options might contribute to instability in different contexts, option B, "when there is a large economic gap between rich and poor," is often a significant factor in the decline of empires. Economic disparity can lead to social unrest, weaken the state, and create conditions ripe for conflict or rebellion, which can ultimately contribute to the fall of an empire.
In the comparison of the Zhou and Qin dynasties, the Qin dynasty's strong centralization and reforms, though initially successful, did lead to significant social tensions and resentment among the populace due to heavy taxation and labor demands, ultimately contributing to its rapid decline.