Which of the following oxoacid of sulphur has
-O-O- linkage ?
(1) ``\ce{H2SO3}``
, sulphurous acid
(2) ``\ce{H2SO4}``
, sulphuric acid
(3) ``\ce{H2S2O8}``
, peroxodisulphuric acid
(4) ``\ce{H2S2O7}``
, pyrosulphuric acid