The solution of the differential equation, for $t > 0, y"(t)+2y'(t)+y(t)=0$ with initial conditions $y(0)=0$ and $y'(0)=1$, is ($u(t)$ denotes the unit step function),
1. $te^{-t}u(t)$
2. $(e^{-t}-te^{-t})u(t)$
3. $(-e^{-t}+te^{-t})u(t)$
4. $e^{-t}u(t)$