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: HOLIDAYS & OBSERVANCES 
Clue: As the Constitutional bicentennial reminded us, the Constitution was signed on this date in 1787
september 17th

Category: WORLD GEOGRAPHY 
Clue: The Lusatian Mountains, in the western Sudeten range, form part of the border between these 2 countries
germany & the czech republic

Category: RIVERS 
Clue: These 2 rivers, each more than 1,000 miles long, rise in the Armenian Plateau in Turkey

the tigris & the euphrates

