The potential energy of a particle of mass 0.1 kg moving along the $X$-axis, is given by $U=5 x(x-4) \mathrm{J}$, where $x$ is in metres. Choose the wrong option.

One end of a spring of force constant $$k$$ is fixed to a vertical wall and other to a body of mass $$m$$ resting on a smooth horizontal surface. There is another wall at a distance $$x_0$$ from the body. The spring is then compressed by $$2 x_0$$ and released. The time taken to stike the wall is