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 = 9x + 2y = 14B. x + y = 142x + y = 9C. x + y = 14x + 2y = 9D. x + y = 92x + y = 14