Given below are two statements:
Statement I : Frenkel defects are vacancy as well as interstitial defects.
Statement II : Frenkel defect leads to colour in ionic solids due to presence of F-centres.

Choose the most appropriate answer for the statements from the options given below:

The coordination number of an atom in a body-centered cubic structure is______

[Assume that the lattice is made up of atoms.]

Given below are two statements. One is labelled as
Assertion $\mathrm{A}$ and the other is labelled as Reason $\mathrm{R}$.
Assertion $\mathrm{A}:$ Sharp glass edge becomes smooth on heating it up to its melting point.

Reason $\mathrm{R}:$ The viscosity of glass decreases on melting.

Choose the most appropriate answer from the options given below.