Gwen read the first 15 pages of a book in a bookstore and decided to buy it. Since she bought it, she has been reading 30 pages per day. For which of these functions does y represent the total number of pages Gwen has read if x represents the number of days since she bought the book?
A. x = 15y + 30
b. x = 30y + 15
c. y = 15x +30
d. y = 30x +15