The number of bridged oxygen atoms present in compound B formed from the following reactions is
$\mathrm{Pb}\left(\mathrm{NO}_3\right)_2 \xrightarrow{673} \mathrm{~A}+\mathrm{PbO}+\mathrm{O}_2$
$\mathrm{A} \xrightarrow{\text { Dimerise }} \mathrm{B}$
