The significance of the "Articles of Confederation" lies in that it is the very first written constitution of the United States. The structure of the "Articles" provides for independence and sovereignity of the states, while Congress appears as a last resort for disputes.
The US started making hydrogen bombs when the USSR succeeded in creating an atomic bomb. The United States used to have the largest bomb, but when the USSR surpassed them, the US decided to create a better bomb (theorized to be hundreds of times larger than any nuclear bomb ever made).
The correct answer is South Carolina.
South Carolina threatened to secede (aka leave) the Union after the Tariffs of 1828 and 1832 because these hurt the trade relationship between South Carolina and European countries like Great Britain. A tariff is a tax on an imported good. This increases the price of foreign goods. When a tariff is put on foreign goods, usually other countries are less likely to trade with different US states.
This is why South Carolina threatened to leave, as these tariffs hurt their trade relations with other countries.
Remember MANIA:
MILITARISM
ALLIANCES
NATIONALISM
IMPERIALISM
ASSASSINATION