Let $Y(s)$ be the unit-step response of a causal system having a transfer function

$G(s)= \frac{3-s}{(s+1)(s+3)}$

that is ,$Y(s)=\frac{G(s)}{s}.$ The forced response of the system is

- $u(t)-2e^{-t}u(t)+e^{-3t}u(t)$
- $2u(t)-2e^{-t}u(t)+e^{-3t}u(t)$
- $2u(t)$
- $u(t)$