I wanna say the answer is- B.
I might be wrong,so if I am,I'm sorry
The event that occurred in the 4th Crusade was the Crusaders attacked Constantinople. The sack of Constantinopea took place in April 1204, during the 4th Crusade. The Crusaders attacked, destroyed and robbed during the assault of Constantinople. (Hope this helps! :)
Because once an indentured servants time is up, you have to let them live their own life, but if you have a slave, you literally own them and keep them until they die if you choose to do so.
Money, resources, soldiers, allies