Prandtl number signifies the ratio of

1. $\frac{Momentum \: Diffusivity}{Thermal \: Diffusivity}$
2. $\frac{Mass \: Diffusivity}{Thermal \: Diffusivity}$
3. $\frac{Thermal \: Diffusivity}{Momentum \: Diffusivity}$
4. $\frac{Thermal \: Diffusivity}{Mass \: Diffusivity}$