In Van der Waals equation $\left( P + \frac{a}{V^2} \right)(V - b) = RT$,what do $a$ and $b$ represent?

• A
  Both $a$ and $b$ represent correction in volume.
• B
  Both $a$ and $b$ represent adhesive force between molecules.
• C
  $a$ represents adhesive force between molecules and $b$ represents correction in volume.
• D
  $a$ represents correction in volume and $b$ represents adhesive force between molecules.