The blades of a windmill sweep out a circle of area $A$. If the wind flows at a velocity $b$ perpendicular to the circle, then the mass of the air of density $\rho $ passing through it in time $t$ is
  • A$Av\rho t$
  • B$2Av\rho t$
  • C$Av^2\rho t$
  • D$\frac{1}{2}Av\rho t$
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