A compound "X", upon reaction with ${\text{H}}_2\text{O}$, produces a colourless gas "Y" with a rotten fish smell. Gas 'Y' is absorbed in a solution of ${\text{CuSO}}_4$ to give ${\text{Cu}}_3{\text{P}}_2$ as one of the products. The compound 'X' will be:

1. A
   ${\text{Ca}}_3{\text{P}}_2$
2. B
   ${\text{NH}}_4\text{Cl}$
3. C
   ${\text{As}}_2{\text{O}}_3$
4. D
   ${\text{Ca}}_3({\text{PO}}_4{)}_2$