The hydrides of which group elements of the periodic table form electron-precise hydrides?

Calcium imide on hydrolysis gives gas $(B)$ which on oxidation by bleaching powder gives gas $(C)$. Gas $(C)$ on reaction with magnesium gives compound $(D)$ which on hydrolysis gives again gas $(B)$. Identify $(B)$,$(C)$ and $(D)$.

In which of the following processes is the highest purity $(99.9 \%)$ hydrogen obtained?

Identify the correct statements from the following:
$(I)$ The formula of Calgon is $Na_6(PO_3)_6$.
$(II)$ Exhausted permutit is regenerated by aqueous $KCl$ solution.
$(III)$ Calcium stearate is soluble in water.
$(IV)$ In ice crystals,each oxygen is surrounded by four oxygen atoms.

When $NaBH_4$ is dissolved in water,

Ionic hydrides are usually