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: WORLD HISTORY 
Clue: He completed vols. 2 & 3 of Karl Marx's "Das Kapital"
friedrich engels

Category: HOLIDAYS AND OBSERVANCES 
Clue: The name of this Jewish holy day means "beginning of the year"
rosh hashanah

Category: POPES 
Clue: Little is known of the second pope who bore this name, but we doubt he sucked his thumb & carried a blanket

linus