Of the following choose the wrong statement

• A

  Internal forces of the system do not affect the motion of centre of mass

• B

  In uniform gravitational field centre of mass and centre of gravity coincide 

• C

  Two balls are thrown simultaneously in air.  The acceleration of the centre of mass of those two balls in air is equal to 'g'.

• D

  The position of centre of mass depends on mass distribution and on the co - ordinate system chosen.