Which was not a method by which the Persians strengthened their empire?

Question

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The Persians did not use a unified, comprehensive law code like Hammurabi's to strengthen their empire. Instead, they relied on other methods such as giving autonomy to provincial administrators, promoting religious toleration, using technology and learning from other cultures, and building roads and highways to connect the far-flung reaches of the empire.

Which Was Not a Method by Which the Persians Strengthened