If $\bar{M}_w$ is the weight average molecular weight and $\bar{M}_n$ is the number average molecular weight of a polymer,the poly dispersity index $(PDI)$ of the polymer is given by

• A
  $\frac{\bar{M}_n}{\bar{M}_w}$
• B
  $\frac{\bar{M}_w}{\bar{M}_n}$
• C
  $\bar{M}_w \times \bar{M}_n$
• D
  $\frac{1}{\bar{M}_w \times \bar{M}_n}$