Food Freezing, Plank's Equation & Freezing Time

In Plank's equation for freezing time ($t_F = \frac{\rho \lambda}{\Delta T} \left[ \frac{P a}{h} + \frac{R a^2}{k} \right]$), for an infinite slab of thickness $a$, the geometric constants $P$ and $R$ are respectively:

Options:

• (A) $P = 1/2,\ R = 1/8$
• (B) $P = 1/4,\ R = 1/16$
• (C) $P = 1/6,\ R = 1/24$
• (D) $P = 1,\ R = 1$