The impulse response functions of four linear systems $\mathrm{S}_{1}, \mathrm{~S}_{2}, \mathrm{~S}_{3}, \mathrm{~S}_{4}$ are given respectively by
$\begin{array}{ll} h_{1}(t)=1 & h_{2}(t)=U(t) \\ h_{3}(t)=\frac{U(t)}{t+1} & h_{4}(t)=e^{3 t} U(t) \end{array}$
Where $\mathrm{U}(t)$ is the unit step function. Which of these systems is time invariant, causal, and stable?
- $\mathrm{S}_{1}$
- $\mathrm{S}_{2}$
- $\mathrm{S}_{3}$
- $\mathrm{S}_{4}$