What is the product formed when $Ca(OH)_2$ reacts with $SO_2$?

Which of the following has the highest electrode potential?

The cheap compound having a high melting point used in a furnace is

Assertion $(A)$: $MgO$, $CaO$, $SrO$, and $BaO$ are insoluble in water.
Reason $(R)$: In aqueous medium, the basic strength of $MgO$, $CaO$, $SrO$, and $BaO$ increases with an increase in the atomic number of the metal.
The correct option among the following is:

$A$ white solid '$A$' on heating gives off a gas which turns lime water milky. The residue is yellow when hot but turns white on cooling. This solid '$A$' is

Chemical $A$ is used for softening temporary hard water. $A$ reacts with sodium carbonate to produce caustic soda. When $CO_2$ is passed through the solution of $A$,it turns milky. What is the chemical formula of $A$?