It was the first multicultural empire in history. It was also the first very large empire. Because of this the Empire had to develop ways of running effectively. Roads were built over great distances. Standardize currency. Localized governing systems (i.e. satrapies). It was also the first time Asian powers attempted to invade Europe.
I think the answer is C :)
The Mexican society wasn't united, since they were constantly fighting with each other. Since soldiers were killed, they also lost military power. When the Spanish arrived, they had a strong military, but also diseases, which killed Mexican soldiers. That made the home society vulnerable.
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The electric light was the invention which allowed color to be projected with increased brightness and clarity.