You will be given a definition of a task first, then some input of the task.
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: BIOLOGY 
Clue: It's the process by which an organism or cell takes in oxygen & expels carbon dioxide
Output:
respiration