Match the following substances in List-$I$ with their uses in List-$II$:
| List-$I$ (Substance) | List-$II$ (Use) |
| :--- | :--- |
| $(A)$ $Na_2O_2$ | $(I)$ Photoelectric cells |
| $(B)$ $D_2O$ | $(II)$ Antacid |
| $(C)$ $Cs$ | $(III)$ Oxidising agent |
| $(D)$ $Mg(OH)_2$ | $(IV)$ Moderator |

$A$ pink colored salt on dry heating becomes blue in colour. The salt may contain:

Match the following items in Column-$I$ with the corresponding items in Column-$II$.

Column-$I$	Column-$II$
$I$. $Na_2CO_3 \cdot 10H_2O$	$A$. Portland cement ingredient
$II$. $Mg(HCO_3)_2$	$B$. Castner-Kellner process
$III$. $NaOH$	$C$. Solvay process
$IV$. $Ca_3Al_2O_6$	$D$. Temporary hardness

Identify the reactions in which,upon heating,a diatomic gas is evolved,leaving behind a metallic residue.
$(A)$ $2NaN_{3(s)} \xrightarrow{\Delta} 2Na_{(s)} + 3N_{2(g)}$
$(B)$ $Ni(CO)_{4(g)} \xrightarrow{250^{\circ}C} Ni_{(s)} + 4CO_{(g)}$
$(C)$ $2KClO_{3(s)} \xrightarrow{\Delta} 2KCl_{(s)} + 3O_{2(g)}$
$(D)$ $2HgO_{(s)} \xrightarrow{\Delta} 2Hg_{(l)} + O_{2(g)}$
$(E)$ $NH_{4}NO_{2(s)} \xrightarrow{\Delta} N_{2(g)} + 2H_{2}O_{(g)}$

Assertion : $s-$Block elements do not occur free in nature.
Reason : $s-$Block elements are highly electropositive in nature.

The metal that gives hydrogen gas upon treatment with both acid as well as base is