The number of dissociable protons in "orthophosphoric acid" is

Decreasing order of number of ionisable hydrogen atoms in the following molecules is

I. $\mathrm{H}_3 \mathrm{PO}_4$

II. $\mathrm{H}_3 \mathrm{PO}_3$

III. $\mathrm{H}_3 \mathrm{PO}_2$

IV. $\mathrm{H}_4 \mathrm{P}_2 \mathrm{O}_6$

When orthoboric acid $\left(\mathrm{H}_3 \mathrm{BO}_3\right)$ is subjected to strong heating, the residue left is

When graphite is heated at $300^{\circ} \mathrm{C}$ with potassium vapour, it forms $\mathrm{C}_8 \mathrm{~K}$ compound that shows one of the following property.