$A$ body of mass $30 \ kg$ moving with a velocity $20 \ ms^{-1}$ undergoes a one-dimensional elastic collision with another ball of the same mass moving in the opposite direction with a velocity of $30 \ ms^{-1}$. After the collision,the velocities of the first and second bodies respectively are:

• A
  $-30 \ ms^{-1}, 20 \ ms^{-1}$
• B
  $30 \ ms^{-1}, 30 \ ms^{-1}$
• C
  $30 \ ms^{-1}, 20 \ ms^{-1}$
• D
  $40 \ ms^{-1}, 15 \ ms^{-1}$