Which of the following conduct electricity in the fused state?

$A$ white solid '$A$' on heating produces a gas that turns lime water milky,and the remaining solid residue is yellow when hot but turns white on cooling. The solid '$A$' is:

The incorrect order of solubility in water is

Nitrates of all metals are:

The correct solubility order is/are:
$I. CaCO_3 > SrCO_3 > BaCO_3$
$II. Li_2CO_3 < Na_2CO_3 < K_2CO_3$
$III. K_2CO_3 < Rb_2CO_3 < Cs_2CO_3$
$IV. Na_2CO_3 > K_2CO_3 > Rb_2CO_3$

$A$ mixture of sodium oxide and calcium oxide is dissolved in water and saturated with excess carbon dioxide gas. The resulting solution is ...... It contains ......... :