The blades of a wndmill sweeps out a circle of area 'A'. If the wind flows at a velocity 'v' perpendicular to the circle (denisity of air = )    A)    the mass of the air passing through it in time 't' is AVt    B)    the kinetic energy of the air is      C)    If the windmill converts 25% of the wind energy into electrical energy and   then the electrical power produced is 4.5 kw