You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: THE 20th CENTURY 
Clue: In the 1940s Franklin Roosevelt coined this term in reference to all the countries allied against the Axis powers
united nations

Category: CIVIL WAR SITES 
Clue: Of the 6 Civil War-related national military parks, the northernmost & southernmost are in these 2 states
pennsylvania & mississippi

Category: ASTRONOMY 
Clue: In 1910 it could be seen with the naked eye; in 1911 a telescope was necessary; by 1912 it was gone

halley\'s comet