Balance of power between the federal and state governments
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The correct answer is "Charlemagne crowned Holy Roman emperor, 800 CE."
The event that belongs in place C on the timeline is "Charlemagne crowned Holy Roman emperor, 800 CE."
In one of the most important events of that time, the pope Leo III crowned Charlemagne as the New Emperor of the Roman Empire. The exact date, December 25, 800 CE. Charlemagne was the King of the Frankish and was the one who could exert the leadership and influence to calm down the situation in Rome. One of the first things Charlemagne did was to improve the education conditions in the Empire.
The Gulf of Tonkin Resolution <span> authorized the president to take "all necessary measures to repel armed attack" in Vietnam.
This resolution had significant consequences for the Vietnam War and beyond that time. In regard to the Vietnam War, it provided the justification for the president, Lyndon Johnson, to escalate US involvement in the war and magnify the number of US troops there by hundreds of thousands. In US foreign policy in general, it represented an increase of the power of the Commander in Chief (the president) to deploy troops without getting formal approval in advance from Congress.
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I also think it is the first one, but I am not entirely sure.