Which of the following is the weakest base among $NaOH$,$Ca(OH)_2$,$KOH$,and $Ba(OH)_2$?

The composition formula of gypsum is

The alkaline earth metal that imparts apple green colour to the Bunsen flame when introduced in it in the form of its chloride is

Which of the following alkaline earth metals does not exhibit colour when heated in a flame?

Identify the correct statements from the following.
$(A)$ $BeSO_4$ is soluble in water.
$(B)$ $BeO$ is an amphoteric oxide.
$(C)$ $CO$ can be obtained by heating $CaCO_3$ at $1070-1270 \ K$.

$Y$ $\xleftarrow{\Delta ,\, 205^\circ C}$ $CaSO_4 \cdot 2H_2O$ $\xrightarrow{\Delta ,\, 120^\circ C}$ $X$. $X$ and $Y$ are respectively