A particle of mass m is projected from the ground with an initial speed u0 at an angle α 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 u0 . The angle that the composite system makes with the horizontal immediately after the collision is
a. 4π
b. 4π+α
c. 2π−α
d. 2π