When burnt in excess of oxygen, sodium forms a compound $X$ and potassium forms a compound $Y$. The magnetic natures of $X$ and $Y$ respectively are

Consider the following reactions.

$$ \begin{array}{r} \mathrm{Cs}+\mathrm{O}_2 \text { (excess) } \rightarrow X \\ \mathrm{Na}+\mathrm{O}_2 \rightarrow Y \end{array} $$

Identify the correct Statement about $X$ and $Y$

Choose the correct statement from the following.

I. In vapour phase $\mathrm{BeCl}_2$ exists as chlorobridge dimer.

II. $\mathrm{BeSO}_4$ is readily soluble in water.

III. BeO is completely basic in nature.

IV. $\mathrm{BeCO}_3$ being unstable, is kept in the atmosphere of $\mathrm{CO}_2$.

V. $\mathrm{BeCO}_3$ is less soluble among all the carbonates of group 2 elements.