(a. a proton of mass m collides with a particle of unknown mass at rest. after the collision, the proton and the unknown particle are seen moving at an angle of

(a. A proton of mass m collides with a particle of unknown mass at rest. After the collision, the proton and the unknown particle are seen moving at an angle of 90° with respect to each other The mass of the unknown particle is (a) m/2 (c) m/3 (d) 2m IISc-2006 7/11