Why did WW1 became a global war? (please answer in a 5-7 sentence paragraph)

The fact that the First World War was a global war was itself the product of a global order, shaped by the European great powers and held together by an embryonic economic system. ... Decisions made in the interest of individual nations also had an effect on the widening of the war from a regional dispute.

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the answer wold be b) travel among nations should be regulated by an international organization

the answer is use of support