The rms speed of the molecules of a gas in a vessel is $400m{s}^{-1}$ . If half of the gas leaks out at constant temperature, the rms speed of the remaining molecules will be

• A

  $800m{s}^{-1}$

• B

  $400\sqrt{3}m{s}^{-1}$

• C

  $400m{s}^{-1}$

• D

  $400\sqrt{2}m{s}^{-1}$