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 consistency, answers should be in all lower cased letters.

Category: ROCKS & MINERALS 
Clue: A mixture containing antimony is sprayed on canvas to render it this
fireproof

Category: LAND "HA"! 
Clue: Ho Chi Minh's remains are on display in a mausoleum in Ba Dinh Square in this Vietnamese city
hanoi

Category: WHEN YOU WISH UPON A CZAR 
Clue: Talk about a bad heir day! Living up to his name in 1581, he murdered his only viable heir (& his namesake, to boot!)

ivan the terrible