$A$ streamline flow of a liquid of density $\rho$ is passing through a horizontal pipe of cross-sectional area $A_1$ and $A_2$ at two ends. If the pressure of the liquid is $P$ at a point where the flow speed is $v$,then the pressure at another point where the flow speed becomes $3v$ is:

• A
  $P-\frac{3}{4} \rho v^2$
• B
  $P-2 \rho v^2$
• C
  $P-3 \rho v^2$
• D
  $P-4 \rho v^2$