Assertion (A) Graphite is used as a dry lubricant in machines which run at high temperatures.

Reason ( $\mathbf{R}$ ) The layers of graphite slip one over the other when pressure is applied.

The correct option among the following is

At $T(\mathrm{~K})$, copper (atomic mass $=63.5 \mathrm{u}$ ) has fce structure with an edge length of $x \AA$. The density of copper (in $\mathrm{g} \mathrm{cm}^{-3}$ ) at that temperature approximately is $\left(N_A=6.0 \times 10^{23} \mathrm{~mol}^{-1}\right)$

A solid has a structure in which ' W ' atoms are located at the corners of a cubic lattice, oxygen atoms at the edge centre and

Na atom at the body centre. The formula of the compound is

KBr has rock salt type structural arrangements and has a density of

$3.70 \mathrm{~g} / \mathrm{cm}^3$. The edge length of the unit cell is approximately [molecular weight of $\mathrm{KBr}=120 \mathrm{~g} / \mathrm{mol}$ ]