Which of the following does not have the dimensions of velocity? (Given ${\mathrm{\epsilon }}_0$ is the permittivity of free space, ${\mathrm{\mu }}_0$ is the permittivity of free space, v is frequency, $\mathrm{\lambda }$ is wavelength, P is the pressure, and $\mathrm{\rho }$ is density, k is wave number, $\mathrm{\omega }$ is the angular frequency)