Math Help - Prealgebra question

Hi! I have a question, and I would like an explanation only, for I already know the answer to the question. Thanks!

If the total cost of x apples is b cents, what is a general formula for the cost, in cents, of y apples?

A. b/xy

B. x/by

C. xy/b

D. by/x

E. bx/y

use unitary method

e.g.
let cost of 5 box is 25 cents .
therefore cost of 1 box =25/5 cents =5 cents
and cost of 30 such boxes = 30 *(5) = 150 cents

total cost of x apples = b cents
therefore cost of one apples = b/x cents

now you can calculate cost of 2 apples or 9 apples or y apples

cost of y apples = yb/x cents

Thanks for answering the question.