A store sells books for $12 each. In the proportional relationship between x, the number of books purchased, and y, the cost per books in dollars” to “y, the total cost of the books in dollars, the constant of proportionality is 12. Which equation shows the relationship between x and y? A: y = 12 - x

B: y = 12x

C: y = 12 + x

D: y = x under 12

P. S: 12/x to be exact