Borax has the molecular formula

$Na_2B_4O_7$ $\xrightarrow{x} H_3BO_3$ $\xrightarrow{\Delta} B_2O_3$ $\xrightarrow{y} B$
Here $X$ and $Y$ are respectively:

Which of the following elements is used in thermometry at high temperatures?

In $B_2H_6$:

Select the correct statements from the following:
$A$) Aluminium liberates $H_2$ gas with dil. $HCl$ but not with aqueous $NaOH$.
$B$) Formula of sodium metaborate is $Na_3BO_3$.
$C$) Boric acid is a weak monobasic acid.
$D$) For thallium,$+1$ state is more stable than $+3$ state.

In the reaction $B_2O_3 + C + Cl_2 \to A + CO$,the $A$ is: