The total number of compounds having at least one bridging oxo group among the molecules given below is
${\mathrm{N}}_2{\mathrm{O}}_3,{\mathrm{N}}_2{\mathrm{O}}_5,{\mathrm{P}}_4{\mathrm{O}}_6,{\mathrm{P}}_4{\mathrm{O}}_7,{\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_5,{\mathrm{H}}_5{\mathrm{P}}_3{\mathrm{O}}_{10},{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_3,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5$