There are 9 books stacked on a shelf. The thickness of each book is either 1 inch or 2 inches. The height of the stack of 9 books is 14 inches. Which system of equations can be used to determine x, the number of 1-inch-thick books in the stack, and y, the number of 2-inch-thick books?


A.) x + y = 14 2x + y = 9

B.) x + y = 14 x + 2y = 9

C.) x + y = 9 x + 2y = 14

D.) x + y = 9 2x + y = 14