Derek has $20 to spend on used books. Hard cover books cost $5 each and paperbacks cost $2 each. Complete the equation which determines the number x of hardcover books and the number y of paperback books he can buy. What are the possible combinations of books that Derek can buy? Select all that apply.
A. (-2,15)
B. (0,10)
C. ( 2,5)
D. (3,5/2)