Which of the Following Was a Reason for the Rise

Which of the following was a reason for the rise of chattel slavery in the Mediterranean city-states after 500 BCE?

A) Individuals voluntarily became slaves for a specified time as an act of religious devotion.
B) The inhabitants of frontier communities in the north militarily overwhelmed the city-states, creating a new volume of slaves.
C) The explosion of money-based market economies led to an ethos in which everything, even human beings, acquired a monetary value.
D) Following an economic collapse, many people were so impoverished that they sold themselves into slavery to pay their debts.