\((P + \frac a{V^2}) (V - b) = RT\) represents the equation of state of some gases. Where P is the pressure, V is the volume, T is the temperature and a, b, R are the constants. The physical quantity, which has dimensional formula as that of \(\frac{b^2}a\), will be:
(1) Bulk modulus
(2) Modulus of rigidity
(3) Compressibility
(4) Energy density
