The degree of dissociation $(\alpha )$ of a weak electrolyte, $A_x{B}_y$ is related to van't Hoff factor $(i)$ by the expression:

1. A
   $\alpha =\frac{i-1}{x+y+1}$
2. B
   $\alpha =\frac{x+y-1}{i-1}$
3. C
   $\alpha =\frac{x+y+1}{i-1}$
4. D
   $\alpha =\frac{i-1}{x+y-1}$