A pencil at a stationery store costs $1, and a pen costs $1.50. Stefan spent $7 at the store. He bought a total of 6 items. Which system of equations can be used to find the number of pencils (x) and pens (y) he bought?
A. x + 1.5y = 7
x + y = 6
B. 1.5x + y = 7
x = 6y
C. x + 2y = 7
6x = 1.5y
D. 2x + y = 7
6x = 1.5y