A particle of mass m is projected from the ground with an initial speed up at an angle a with the horizontal. At the highest point of its trajectory, it makes a completely inelastic collision with another identical particle, which was thrown vertically upward from the ground with the same initial speed uo. The angle that the composite system makes with the horizontal immediately after the collision is (A) TU 4 (B) TU 4 + TU (C) - 4 (D) 2