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: PEOPLE IN HISTORY 
Clue: Executed in 1587, she was the last reigning queen of Scotland
mary, queen of scots

Category: AMERICAN INDIANS 
Clue: When this chief, Pocahontas' father, died in 1618, he was succeeded by his brother Opitchapam
powhatan

Category: CARBON CREDITS 
Clue: This gas, CO, prevents hemoglobin from supplying oxygen to the body

carbon monoxide