What is atomic radius of an element if it crystallises in BCC structure with edge length of unit cell 287 pm?

Identify the product ' B ' in the following reaction sequence.

$$\mathrm{C}_2 \mathrm{H}_5-\mathrm{Br} \xrightarrow[\text { Dry ether }]{\mathrm{Mg}} \mathrm{~A} \xrightarrow{\mathrm{CH}_3 \mathrm{OH}} \mathrm{~B}$$

Identify the correct statement for following reaction.

$$3 \mathrm{Mg}+\mathrm{N}_2 \longrightarrow \mathrm{Mg}_3 \mathrm{~N}_2$$

Which of the following cannot be used as standard solution for determination of cell constant of conductivity cell?