Before the Turkish Capture of Constantinople

Before the Turkish capture of Constantinople,

A) Murad I defeated the Serbs at the Battle of Kosovo.
B) the Balkan population refused to assist Ottoman efforts in any way.
C) Morocco was the only Mediterranean area controlled by the Spanish conquistadors.
D) the last Byzantine emperor became a Lutheran in a bid for European support.
E) Pope Innocent XII personally led a crusade to save the city for Chritendom.