The periodic time of a S.H.O. oscillating about a fixed point is $2 \mathrm{~s}$. After what time will the kinetic energy of the oscillator become $25 \%$ of its total energy?
(A) $1 / 12 \mathrm{~s}$
(B) $1 / 6 \mathrm{~s}$
(C) $1 / 4 \mathrm{~s}$
(D) $1 / 3 \mathrm{~s}$.