If the displacement of a body of mass $m$ is given by $x=a \sin \omega t$, then its kinetic energy is
(a) $\frac{1}{2} \operatorname{ma}^{2} \omega^{2}[1+\cos \omega \mathrm{t}]$
(b) $\frac{1}{4} \operatorname{ma}^{2} \omega^{2}[1+\sin 2 \omega \mathrm{t}]$
(c) $\frac{1}{4} \operatorname{ma}^{2} \omega^{2}[1+\cos 2 \omega \mathrm{t}]$
(d) $\frac{1}{4} \operatorname{ma}^{2} \omega^{2} \sin ^{2} \omega \mathrm{t}$