Which of the following sulfate salts has the highest solubility in water?

The halides of alkaline earth metals form hydrates such as $MgCl_2 \cdot 6H_2O$,$CaCl_2 \cdot 6H_2O$,$BaCl_2 \cdot 2H_2O$,and $SrCl_2 \cdot 2H_2O$. This indicates that the halides of group $2$ elements are:

Among the following,the basic oxide is .....

Which of the following sulphates has the highest solubility in water?

The highly efficient method of obtaining beryllium is

$CaCO_3$ $\xrightarrow[excess \ H_2O]{CO_2 \ in} A$ $\xrightarrow{\Delta} B$
Identify $A$ and $B$.