The correct sequence of decreasing number of $$\pi $$-bonds in the structures of H₂SO₃, H₂SO₄ and H₂S₂O₇ is :

A metal ‘M’ reacts with nitrogen gas to afford ‘M₃N’. ‘M₃N’ on heating at high temperature gives back ‘M’ and on reaction with water produces a gas ‘B’. Gas ‘B’ reacts with aqueous solution of CuSO₄ to form a deep blue compound. ‘M’ and ‘B’ respectively are :

The number of S = O and S − OH bonds present in peroxodisulphuric acid and pyrosulphuric acid respectively are :

The products obtained when chlorine gas reacts with cold and dilute aqueous NaOH are :