$A$ 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:

Which of the following is $NOT$ obtained when metal carbides react with $H_2O$?

Match the following compounds (Column-$I$) with their uses (Column-$II$):

Column-$I$	Column-$II$
$I. Ca(OH)_2$	$A$. Casts of statues
$II. NaCl$	$B$. White wash
$III. CaSO_4 \cdot \frac{1}{2}H_2O$	$C$. Antacid
$IV. CaCO_3$	$D$. Washing soda preparation

Select the correct statement$(s)$.

The correct statements among the following are:
$(A)$ $BeH_2$ and $MgH_2$ are polymeric in nature.
$(B)$ $LiH$ is unreactive to oxygen at moderate temperatures.
$(C)$ $BeH_2$ and $MgH_2$ possess significant covalent character.
$(D)$ The stability of alkali metal hydrides follows the order $LiH < NaH < KH < RbH < CsH$.

Write a note on $s$-block elements of the periodic table.