In the system shown in the figure, the input is the force $f$ and the output is the displacement $x_A$ of point $A.$ When $x=x_A,$ the spring is at its free length.

$\frac{x_A(s)}{F(s)}=\frac{k}{(mc{s}^3+mk{s}^2+kc)}$

$\frac{x_A(s)}{F(s)}=\frac{s}{k(mc{s}^2+mks+kc)}$

$\frac{x_A(s)}{F(s)}=\frac{k}{s(mc{s}^2+mks+kc)}$

$\frac{x_A(s)}{F(s)}=\frac{ks}{mc{s}^2+mks+kc}$

None of the above