Consider the cellular precipitation $($i$.$e$.,$ eutectoid transformation$)$ of $-Fe$ and $F{e}_{3}C($cementite$)$ from super$-$saturated $-$Fe $($austenite phase$),$ for which we assume the phase transformation $($i$.$e$.,$ the growth of interface$)$ occurs by diffusion through either $$ or $.$ It was observed that the growth rate of the interface is $5\frac{}{sec}$ when the inter$-$lamellar spacing $($i$.$e$.,$ the wavelength, $)$ is $=100nm,$ and the growth rate becomes $0\mathrm{.}6n\frac{m}{sec}$ when the inter$-$ lamellar spacing is $=400nm.($a$)$ What is the maximum of the growth rate and $($b$)$ at what inter$-$lamellar spacing?