A particle of mass $m$ is attached to a spring (of spring constant $k$) and has a natural angular frequency $\omega_0$. An external force $F(t)$ proportional to $\cos \omega t ( \omega \neq \omega_0)$ is applied to the oscillator. The time displacement of the oscillator will be proportional to
( A ) $\frac{1}{m (\omega_0^2 +\omega^2)}$
( B ) $\frac{1}{m (\omega_0^2 - \omega^2)}$
( C ) $\frac{m}{\omega_0^2 - \omega^2}$
( D ) $\frac{m}{\omega_0^2 + \omega^2}$