A particle of mass $m$ initially at rest, is acted upon by a variable force $F$ for a brief interval of time $T$. It begins to move with velocity $u$ after the force stops acting. $F$ is as a function of time shown in the graph. The velocity o $[$ the particle is:
(a) $u-\frac{\pi F_{e}^{2}}{2 m}$
(b) $u-\frac{\pi T^{2}}{8 m}$
(c) $u-\frac{\pi F_{n} T}{4 m}$
(d) $u-\frac{F_{n} T}{2 m}$