How many of the following drying agents that cannot be used for drying ammonia gas are 

a) CaO       b) ${\mathrm{P}}_2{\mathrm{O}}_5$     c) ${\mathrm{CaCl}}_2$   d) $\mathrm{Conc}.{\mathrm{H}}_2{\mathrm{SO}}_4$  

CaO is basic and is a suitable drying  agent for ammonia.${\mathrm{P}}_2{\mathrm{O}}_5$, Fused  ${\mathrm{CaCl}}_2$ and conc.${\mathrm{H}}_2{\mathrm{SO}}_4$  are unsuitable for drying ammonia as they react with  ${\mathrm{NH}}_3$

$6{\mathrm{NH}}_3+{\mathrm{P}}_2{\mathrm{O}}_5+3{\mathrm{H}}_2\mathrm{O}\to 2{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$

Fused ${\mathrm{CaCl}}_2+{\mathrm{NH}}_{3\left(\mathrm{g}\right)}\to {\mathrm{CaCl}}_2.4{\mathrm{NH}}_3$  (solid) 

or ${\mathrm{CaCl}}_2+{\mathrm{NH}}_{3\left(\mathrm{g}\right)}\to {\mathrm{CaCl}}_2.8{\mathrm{NH}}_3$ (solid)

$2{\mathrm{NH}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$