A system with a frequency response $H(j)=1$ has an output function in the form of

$y(t)=e^{-3t}u(t)-e^{-4t}u(t).$ Which one of the following is the input function $(x(t))$ for this system?

a$.$

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

b$.$

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

c$.$

$e^{-4t}u(t)-e^{-3t}u(t)$

d$.$

$e^{-3t}u(t)-e^{-4t}u(t)$

$e^{-4t}u(t)$