TASK DEFINITION: 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.
PROBLEM: Category: HEAVY MACHINERY 
Clue: By definition it's a machine that gives a surface an even degree of slope

SOLUTION: grader

PROBLEM: Category: MEAN GIRLS 
Clue: This supernatural TV series was full of mean girls: Cordelia, the insane vampire Drusilla & Glory (a sort of evil god)

SOLUTION: buffy the vampire slayer

PROBLEM: Category: ARE THE STARS OUT TONIGHT? 
Clue: NASA says the next best windows for man to visit this planet & keep the trip short occur in 2003, 2018 & 2020

SOLUTION:
mars