Niobium crystallises in body centred cubic structure. If density is $$8.55 \mathrm{~g} \mathrm{~cm}^{-3}$$, then the atomic radius of niobium is (atomic mass of niobium $$=93 u$$ )

A solid has a structure in which W atoms are located at the corners of a cubic lattice. O atoms at the centre of edges and Na atom at centre of the cube. The formula for the compound is

Assertion (A) Graphite is an example of tetragonal crystal system.

Reason (R) For a tetragonal system, $$a=b \neq c$$ and $$\alpha=\beta=90^{\circ}, \gamma=120^{\circ}$$.