$(12$ points$)$ Choose the most relevant quantity from the followings:

Similarity variable $,$ Error function erf$(x),Bi,Pr,,Re,$ Mesh Fourier number $,$ Critical length $x_{cr},$ Skin friction coefficient $C_f,$ Local heat transfer coefficient $h_x,$ Dynamic viscosity $,$

Kinematic viscosity $v,$ Thermal diffusivity $,$ Volumetric heat generation $e_{\text{g}\text{e}\text{n}}^{},$

Velocity boundary layer thickness $,$ Thermal boundary laver thickness ${}_T,$ Film temperature $T_f$

$($a$)$ The ratio of convection at the surface of the solid to conduction within the solid body.

$($b$)$ The ratio of the molecular diffusivity of momentum to the molecular diffusivity of heat.

$($c$)$ The quantity of the dynamic viscosity divided by the density.

$($d$)$ The quantity that is used for stability criterion for Explicit Method of numerical $1-$D heat conduction

$($e$)$ The dimensionless convection heat transfer coefficient.

$($f$)$ The vertical distance from the surface of a horizontal plate at which $u=0\mathrm{.}99V,$ where $u$ is the velocity in the horizontal direction and $V$ is the free stream velocity.