Where Did the Shang Kings Obtain the Slaves That Provided

Where did the Shang kings obtain the slaves that provided them with one of their most important sources of revenue?

A) They were often men and women taken as war captives.
B) They were people who were in debt to the king.
C) They were orphans who were taken by the king and sold into slavery.
D) They were the king's former concubines, who were often sold into slavery.