Which innovation promoted trade and contributed to the unity of the persian empire?

Use of standardized metal coins.

Explanation:

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The use of standardized metal coins was the main innovation that allowed the Persian Empire to become more unified. As many people shared such coins, they began to see themselves as members of the same society. Moreover, these coins allowed trade to expand and become more efficient.

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they were citizens but they could not vote and they did could not get involved into a political situation during that time.