Answer:
The domino effect.
Explanation:
The Domino effect is the theory that when one country falls to communism then more will soon follow, one by one. This is what got us into the Cold War and all conflicts surrounding it, including the Korean and Vietnam War.
Montezuma II ruled over the Aztec Empire during the Spanish Conquest. He was the 9th Aztec Emperor and he led and expanded them to its greatest size. But it was under his rule that the Empire fell apart.
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The assassination of Archduke Franz Ferdinand that lead to the devastating, destructive period known as WWI.
The answer is D to get more people involved in the voting process