Water flows steadily through a horizontal pipe of variable cross-section. If the pressure of water is P at a point where flow speed is v, the pressure at another point where the flow
speed is 2v, is : (take density of water as $\mathrm{\rho }$) 

• A

  $\mathrm{P}-\frac{3{\mathrm{\rho v}}^2}{2}$

• B

  $\mathrm{P}-\frac{{\mathrm{\rho v}}^2}{2}$

• C

  $\mathrm{P}-\frac{-3{\mathrm{\rho v}}^2}{4}$

• D

  $\mathrm{P}-{\mathrm{\rho v}}^2$