A converging-diverging nozzle with an area ratio of 2.75 is attached to a reservoir containing air at 250 psia and 500 R. Calculate: a) the maximum back pressure posible for the flow to be choked, b) the back pressure resulting in a normal shock just at the exit plane, and c) the back pressure which would result in supersonic flow with no oblique shock or expansion wave structure downstream of the nozzle.