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: PRESIDENTIAL FIRSTS 
Clue: 1st president to refuse a 3rd term
george washington

Category: HOW DO THEY WORK? 
Clue: Many car salesman work on "straight" this, though you're better off buying from one who's on salary
commission

Category: MEDICAL MILESTONES 
Clue: Louise Brown, the first human conceived by in vitro fertilization, is better known as the 1st this "baby"

test tube baby