In the kinetic theory of gases,which of these statements is/are true?
$(i)$ The pressure of a gas is proportional to the mean speed of the molecules.
$(ii)$ The root mean square speed of the molecules is proportional to the pressure.
$(iii)$ The rate of diffusion is proportional to the mean speed of the molecules.
$(iv)$ The mean translational kinetic energy of a gas is proportional to its kelvin temperature.

• A
  $(ii)$ and $(iii)$ only
• B
  $(i), (ii)$ and $(iv)$ only
• C
  $(i)$ and $(iii)$ only
• D
  $(iii)$ and $(iv)$ only