Find the derivative.

$y=\frac{e^{-x}+1}{e^x}$

$A.\frac{dy}{dx}=\frac{e^x-2}{e^{2x}}$

$B.\frac{dy}{dx}=\frac{-e^x-2}{e^{2x}}$

$C.\frac{dy}{dx}=\frac{-e^x+2}{e^{2x}}$

$D.\frac{dy}{dx}=\frac{e^x+2}{e^{2x}}$