A bookstore costs $90 a day to keep open, and spends $12 for each book it sells. The store charges $18 for each book it sells. If n represents the number of books sold, which equation represents the cost function for this bookstore?
O A. C = 90n + 15
O B. C = 12n + 90
O C. C = 18n + 90
O D. C = 90n + 12​