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The assassination of Archduke Francis Ferdinand was the catalyst that started World War I because

AHis death brought to a head the tensions underlying the alliances in eastern and western Europe
BHe had been a unifying force between Serbia and the Austro-Hungarian empire
CHis death caused Germany to rush to the defense of Serbia
DHis death ended plans for national self-determination within the Austro-Hungarian empire

Verified Answer :

The assassination of Archduke Francis Ferdinand was the catalyst that started World War I because "a. His death brought to a head the tensions underlying the alliances in eastern and western Europe," since it was actually the complicated system of alliances that really "set off" WWI. 

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