If the extension of the spring is $x_{0}$ at time $t$, then the displacement of the second block at this instant is
(a) $\frac{F l^{2}}{2 m}-x_{0}$
(b) $\frac{1}{2}\left(\frac{F l^{2}}{2 m}+x_{0}\right)$
(c) $\frac{1}{2}\left(\frac{2 F t^{2}}{m} \quad x_{o}\right)$
(d) $\frac{1}{2}\left(\frac{F t^{2}}{2 m} \quad x_{0}\right)$