By the time of darius, the persian kings had created a. so many assassination plots that the empire was weakened, leading to conquest by genghis khan. b. such high taxes that loyalty to the empire began to decline. c. hundreds of varieties of the game of checkers d. a standing army known as the immortals because whenever a member was killed, he was immediately replaced.

d. a standing army known as the Immortals because whenever a member was killed, he was immediately replaced.

Explanation:

During the Achaemenid Empire, the Persian kings had created a standing army called Immortals because of its heavily-armed infantry consist of short spears, swords, wicker shields, bow and arrow. The army was consist of 10,000 soldiers.  If wounded or killed soldiers replaced. Soldiers during the Achaemenid empire serves as a standing army and as an Imperial Guard.

written by alan ward and published in 2003. ward is a historian and was a professor at the university of connecticut.

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it required the unanimous approval of the states before the congress could enact any measure.