Magnesium wire is heated in the atmosphere of $CO_2$ because

Which of the following metals forms a saline hydride?

$A + H_2O \to NaOH$; $A$ $\xrightarrow[400 \ ^oC]{O_2} B$ $\xrightarrow[25 \ ^oC]{H_2O} NaOH + O_2$
$B$ is used for oxygenating in submarines. $A$ and $B$ are:

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Lithium salts are hydrated.
Reason $(R):$ Lithium has higher polarising power than other alkali metal group members.
In the light of the above statements,choose the most appropriate answer from the options given below:

The frequency of the radiation emitted by alkali metals in the flame test follows the order:

Sodium metal reacts with $Al_2O_3$ at high temperature to give a sodium compound $X$. Compound $X$ reacts with carbon dioxide in water to give $Y$. What is $Y$?