\textbf{168.} A body of mass $m$ is connected to a spring with constant $k$ and oscillates. The period of oscillation is $0.9\pi$ seconds. If the mass of the body decreases by $190\,\text{g}$ and the period of oscillation becomes $0.09\pi$ seconds, $k$ is how many newtons per meter?
(1) $2$ \qquad (2) $4$ \qquad (3) $20$ \qquad (4) $40$