Which from following polymers contain - CO - NH - linkage in it?

Identify the product of ozonolysis of but-2-ene from following:

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.

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