Your friend has $100 when he goes to the fair. He spends $10 to enter and $20 on food. Rides at the fair cost $2 per ride. Which function can be used to determine how much money he has left over after x rides?

A.) f(x) = -2x + 70
B.) f(x) = 2x + 70
C.) f(x) = -2x - 70
D.) f(x) = -30x + 100