Consider a gas with density ρ and C as the root mean square velocity of its molecules contained in a volume. If the system moves as a whole with velocity ν, then the pressure exerted by the gas is
$\dfrac{1}{3}\rho C^2$
$\dfrac{1}{3}\rho (C + v)^2$
$\dfrac{1}{3}\rho (C - v)^2$
$\dfrac{1}{3}\rho (C^2 - v^2)$