The solution of the ordinary differential equation

$\frac{dy}{dx}+3y=1$, subjects to the initial condition $y=1$ at $x=0$, is

1. $\frac{1}{3}\left ( 1+2e^{-x/3} \right )$
2. $\frac{1}{3}\left ( 5-2e^{-x/3} \right )$
3. $\frac{1}{3}\left ( 5-2e^{-3x} \right )$
4. $\frac{1}{3}\left ( 1+2e^{-3x} \right )$