A particle at the end of a spring executes simple harmonic motion with a period $t_1$, while the corresponding period for another spring is $t_2$. If the period of oscillation with the two springs in series is $t$, then
( A ) $T^2 = t_1^2 + t_2^2$
( B ) $T = t_1+t_2$
( C ) $T^{-1} = t_1^{-1 }+ t_2^{-1}$
( D ) $T^{-2} = t_1^{-2 }+ t_2^{-2}$