In the equation $$X + \frac{a}{Y^2}\left[Y - b\right] = RT$$, $$X$$ is pressure, $$Y$$ is volume, $$R$$ is universal gas constant and $$T$$ is temperature. The physical quantity equivalent to the ratio $$\frac{a}{b}$$ is:

A

Pressure gradient

B

Energy

C

Impulse

D

Coefficient of viscosity