D. All answers are correct.
The slaves included the captives from war (raids from other regions), debtors of large amounts of money and criminals.
The slaves was vital to the economy of the empire. They carried out the heavy work, served as soldiers, craftsmen, and some served as royal advisers.
Tribute is the giving of a gift or stated sum <span>as that paid by a subject to a <span>sovereign.</span></span>
