Let Y(s) be the unit-step response of a causal system having a transfer function
$$G\left( s \right) = {{3 - s} \over {\left( {s + 1} \right)\left( {s + 3} \right)}}$$
That is, $$Y\left( s \right) = {{G\left( s \right)} \over s}.$$   The forced response of the system is

A. u(t) - 2e^-t u(t) + e^-3t u(t)

B. 2u(t)

C. u(t)

D. 2u(t) - 2e^-t u(t) + e^-3t u(t)

Answer: Option A