If the extension of the spring is $x_{10}$ at time $t$, then the displaccment of the first block at this instant is:
(a) $\frac{1}{2}\left(\frac{F t^{2}}{2 m}+x_{a}\right)$
(b) $-\frac{1}{2}\left(\frac{F t^{2}}{2 m}+x_{0}\right)$
(c) $\frac{1}{2}\left(\frac{F t^{2}}{2 m} \quad x_{0}\right)$
(d) $\frac{F t^{2}}{2 m} \mid x_{v}$