If $|x| < 1$ and $y = x - \frac{x^2}{2} + \frac{x^3}{3} - \frac{x^4}{4} + \ldots$,then $x$ is equal to :

• A
  $y + \frac{y^2}{2!} + \frac{y^3}{3!} + \ldots$
• B
  $y - \frac{y^2}{2!} + \frac{y^3}{3!} - \frac{y^4}{4!} + \ldots$
• C
  $y + \frac{y^2}{2} + \frac{y^3}{3} + \ldots$
• D
  $y - \frac{y^2}{2} + \frac{y^3}{3} - \frac{y^4}{4} + \ldots$