An equation of state is explicit in pressure $p$ and cubic in the specific volume $v$. At the critical point $‘c’$ , the isotherm passing through $‘c’$ satisfies

1. $\frac{\partial p}{\partial v} < 0, \frac{\partial^2 p}{\partial v^2} = 0$
2. $\frac{\partial p}{\partial v} > 0, \frac{\partial^2 p}{\partial v^2} > 0$
3. $\frac{\partial p}{\partial v} = 0, \frac{\partial^2 p}{\partial v^2} > 0$
4. $\frac{\partial p}{\partial v} = 0, \frac{\partial^2 p}{\partial v^2} = 0$