If $\rho$ is the density and $\eta$ is the coefficient of viscosity of a fluid which flows with a speed $v$ in a pipe of diameter $d$,the correct formula for Reynolds number $R_{e}$ is ..............

• A
  $R_{e}=\frac{\eta d}{\rho v}$
• B
  $R_{e}=\frac{\rho v}{\eta d}$
• C
  $R_{e}=\frac{\rho vd}{\eta}$
• D
  $R_{e}=\frac{\eta}{\rho vd}$