Which $ONE$ of the following is a linear non-homogeneous differential equation, where $x$ and $y$ are the independent and dependent variables respectively?
- $\frac{dy}{dx}+xy= e^{-x}$
- $\frac{dy}{dx}+xy= 0$
- $\frac{dy}{dx}+xy= e^{-y}$
- $\frac{dy}{dx}+ e^{-y}= 0$