The number of $S-S$ bonds in sulphur trioxide trimer $S_3O_9$ is

The pairs among $A = [SO_3^{2-}, CO_3^{2-}]$, $B = [O_2^{2-}, F_2]$, $C = [CN^-, CO]$, $D = [NH_3, H_3O^+]$ and $E = [MnO_4^{2-}, CrO_4^{2-}]$ that do not have similar Lewis dot structures are:

Observe the following structure. The formal charges on the atoms $1, 2, 3$ respectively are:

The order of the average bond length of the given bonds is

$BCl_3$ does not exist as a dimer,but $BH_3$ exists as a dimer $(B_2H_6)$ because:

The species having the maximum $Cl-O$ bond order is: