Teacher: 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.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Category: HOMOPHONIC PAIRS 
Clue: It's a notation on a percussion store to clash
Solution: cymbal symbol
Reason: "Cymbal" and "symbol" both have the same pronunciations but different meanings, hence they are homophonic pairs. A symbol is the notation and a cymbal is a percussion instrument that clashes and rings.

Now, solve this instance: Category: SCIENTIFIC FIRSTS 
Clue: The first object in our solar system discovered by telescope was not a planet but one of these
Student:
a moon