Joe's budget constraint equals 500 = 2F + 100S, where $500 is Joe's income, $2 is the price of food (F) and $100 is the price of shelter (S). How much food can Joe buy if he buys one unit of shelter?
A) 2 units
B) 200 units
C) 250 units
D) 400 units
Answer: 200 units