A disc is free to rotate about a smooth horizontal axis passing through its centre of mass. A particle is fixed 
at the top of the disc. A slight push is given to the disc and it starts rotating. During the process,

• A

  only mechanical energy is conserved

• B

  only angular momentum (about the axis of rotation) is conserved

• C

  Both mechanical energy and angular momentum are conserved

• D

  Neither the mechanical energy nor the angular momentum are conserved