The given graph represents the variation of $Z$ (compressibility factor = $\frac{PV}{nRT}$) versus $P$,for three real gases $A, B$ and $C$. Identify the only incorrect statement.

• A
  For the gas $A, a=0$ and its dependence on $P$ is linear at all pressure.
• B
  For the gas $B, b=0$ and its dependence on $P$ is linear at all pressure.
• C
  For the gas $C$,which is a typical real gas for which neither $a$ nor $b=0$. By knowing the minima and the point of intersection with $Z=1$,$a$ and $b$ can be calculated.
• D
  At high pressure,the slope is positive for all real gases.