A ball of mass m strikes an identical ball at rest with speed of 10 m/s. After collision which is assumed perfectly elastic the first ball is found to move with speed 8 m/s. Speed of $2 ^{nd}$ ball and angle between the paths of two balls after collision is

Question

$$(a)\;4\; m/s; 45 ^{\circ} \quad (b)\;6\; m/s; 45 ^{\circ} \quad (c)\;4\; m/s; 90 ^{\circ} \quad(d)\;6\; m/s; 90 ^{\circ}$$