A system of two blocks A and B are connected by an inextensible massless string as shown. The pulley is massless & frictionless. Initially the system is at rest when a bullet of mass 'm' moving with a velocity 'u' as shown hits the block 'B' and gets embeded into it. The impulse imparted by tension force to the block of mass '3 m' is

(1) $\frac{5\mathrm{mu}}{4}$                   (2) $\frac{4\mathrm{mu}}{5}$                     (3) $\frac{2\mathrm{mu}}{5}$                       (4) $\frac{3\mathrm{mu}}{5}$