The best answer would be B. :)
Answer:
to ban slavery in all land gained from the Mexican War.
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! :)
1978 is the answer comrade