Identify thermoplastic polymer from following.

Calculate radius of fourth orbit of $\mathrm{B}^{4+}$ ion.

Identify ' B ' in the following conversion.

$$\mathrm{CH}_3-\mathrm{I} \xrightarrow{\mathrm{KCN}} \mathrm{~A} \xrightarrow{\mathrm{Na} / \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}} \mathrm{~B}$$

What is the number of lone pair of electrons on central halogen atom in BrF$_3$?