If Good $x$ Is Measured Along the Horizontal Axis and Good $y$

If good $x$ is measured along the horizontal axis and good $y$ is measured along the vertical axis, then the slope of the budget constraint can be expressed as which of the following?

A) $- \frac { P _ { x } } { P _ { y } }$
B) $- \frac { I } { P _ { y } }$
C) $- \frac { l } { P _ { x } }$
D) $- \frac { P _ { y } } { P _ { x } }$