Kjeldahl's method cannot be used for the estimation of nitrogen in which compound?

Which of the following compounds is most reactive in $\mathrm{S}_{\mathrm{N}} 1$ reaction?

The coagulating power of electrolytes having ions $\mathrm{Na}^{+}, \mathrm{Al}^{3+}$ and $\mathrm{Ba}^{2+}$ for $\mathrm{As}_2 \mathrm{S}_3$ sol increases in the order

If three elements $A, B, C$ crystalise in a cubic solid lattice with $B$ atoms at the cubic centres, $C$ aton at the centre of edges and A atoms at the corners, then formula of the compound is