Please help me understand why in addition to the answer

At $800C$ Fe has an atomic radius of $0\mathrm{.}126nm.$

What is the lattice parameter for iron at $800C?$

A$.0\mathrm{.}10-0\mathrm{.}20nm$

B$.0\mathrm{.}20-0\mathrm{.}30nm$

C$.0\mathrm{.}30-0\mathrm{.}40nm$

D$.0\mathrm{.}40-0\mathrm{.}50nm$