Group $2$ hydrides with significant covalent character is/are:

From which of the following is the hydration energy of $Mg^{2+}$ larger?

Which element from the following exhibits a common oxidation state of $+2$?

Which alkaline earth metal carbonate decomposes most easily?

Consider the following reaction sequence:
$CaO + CO_2 \to A$
$A \xrightarrow{\Delta} B + CO_2$
$B + H_2O \to C$
$C + CO_2 \to D$
$D + H_2O + CO_2 \to E$
Identify compounds $D$ and $E$.

Which of the following statements is not correct?