A compound A when treated with a dilute mineral acid gives a gas which when passed through a solution of B regenerates A. Further, a gas C that is obtained from the addition of concentrated $$\mathrm{HCl}$$ to $$\mathrm{KMnO_4}$$ crystals is used to react with B to give D. Identify A, B, C and D.

A

$$\mathrm{Na_2CO_3,NaOH,O_2,Na_2O}$$

B

$$MgCO_{3},Mg(OH)_{2},OCl_{2},Mg(OCl)_{2}$$

C

$$CaCO_{3},Ca(OH)_{2},Cl_{2},CaOCl_{2}$$

D

$$Al_2(CO_3)_3,Al(OH)_3,Cl_2,Al_2O_3$$