The solution of the equation $\dfrac{dQ}{dt} +Q =1$ with $Q=0$ at $t=0$ is

1. $Q(t)=e^{-t}-1$
2. $Q(t)=1+ e^{-t}$
3. $Q(t)=1 -e^t$
4. $Q(t)=1- e^{-t}$

Answer: