What is the mixture of $MgCl_2$ and $MgO$ called?

$A$ 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:

Choose the correct statement and mark the correct option given below:
$(a)$ $AgF$ is soluble and colourless in aqueous medium.
$(b)$ For $LiF$,solubility and melting point are both lower as compared to $NaF$.
$(c)$ Solubility of $Rb_2SO_4$ is less as compared to $BaSO_4$.
$(d)$ If $AgI$ is added to water,the colour of water will turn yellow from colourless.

Match the elements given in Column-$I$ with the properties mentioned in Column-$II$ :

Column-$I$	Column-$II$
$A$. $Li$	$1$. Insoluble sulphate
$B$. $Na$	$2$. Strongest monoacidic base
$C$. $Ca$	$3$. Most negative $E^{\circ}$ cell value among alkali metals
$D$. $Ba$	$4$. Insoluble oxalate
	$5$. $6s^2$ outer electronic configuration

Which of the following alloys is used for filling dental cavities?

Copper becomes green when exposed to moist air for a long period. This is due to