The Fick's first law of diffusion is derived from the following equation:

$J=-\frac{a^{2}f}{6}*e^{-\frac{G^A}{R}T}*\frac{dc(x)}{dx}$

$($a$)$ Describe the physical meaning of a$,$ f$,G^A?$

$($b$)$ Write the formula and corresponding units for diffusion constant $D,$ jumping frequency, and diffusion length.

$($c$)$ Write the equation for the Fick's second law of diffusion in $3$D$.$