Final answer:
The piece of evidence that best supports the claim that Alliances were the biggest cause of WWI is the fact that if one nation went to war, all of them might find themselves in a fight.
Step-by-step explanation:
The piece of evidence that best supports the claim that Alliances were the biggest cause of WWI is option D: "...if one nation went to war, all of them might find themselves in a fight." (slide 12)
This evidence demonstrates that the complex system of alliances created a situation where a conflict between two nations could quickly escalate into a world war involving multiple countries.
For example, when Austria-Hungary declared war on Serbia in 1914, their alliance with Germany pulled Germany into the conflict, which in turn triggered the involvement of the Triple Entente, leading to the start of WWI.
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