How many of the following molecules do not contain peroxide linkage?

${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5, {\mathrm{H}}_2{\mathrm{SO}}_5, {\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_8, {\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_8, {\mathrm{H}}_3{\mathrm{PO}}_5, {\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$

${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5$ and ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$ do not contain peroxide linkage.

If the chemical ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5$ and ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$ does not have two oxygen atoms joined by a single bond, it possesses peroxy linkage. The Peroxy linkage is denoted by the symbol −O−O−. One peroxy linkage exists in this molecule.