What turn world war i into a global war?

World War I became a global conflict for several reasons. One reason was that many countries around the war were involved in some way with the war. The alliance system helped spread the war quickly. The two main alliances during the war were the Allied Powers and the Central Powers. While the emphasis is given to the main powers in each alliance, there were smaller countries allied with each side.
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Ithink the answer is a :