If $u(t)$ is a unit step function, the solution of the differential equation $m \frac{d^{2} x}{d t^{2}}+k x=u(t)$ in Laplace domain is
(A) $\frac{1}{s\left(m s^{2}+k\right)}$
(B) $\frac{1}{m s^{2}+k}$
(C) $\frac{s}{m s^{2}+k}$
(D) $\frac{1}{s^{2}\left(m s^{2}+k\right)}$
If $u(t)$ is a unit step function, the solution of the differential equation $m \frac{d^{2} x}{d t^{2}}+k x=u(t)$ in Laplace domain is
(A) $\frac{1}{s\left(m s^{2}+k\right)}$
(B) $\frac{1}{m s^{2}+k}$
(C) $\frac{s}{m s^{2}+k}$
(D) $\frac{1}{s^{2}\left(m s^{2}+k\right)}$