Which Battle was the turning point in Eastern Europe?

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).

the correct answer would be a. i think tara will practice for her test because she realizes her mistake

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d is the answer hope this