Birds have negative pressure breathing, but it differs from that of mammals and is more efficient because of which of the following reasons?

Birds have negative pressure breathing, but it differs from that of mammals and is more efficient because of which of the following reasons?A) The bird's mouth movements are able to force air into the lungs.B) The tidal volume in birds is much larger than in a comparably sized mammal.C) The maximum PO2 is significantly higher in bird lungs.D) The flow of air in a bird's lungs is from posterior to anterior.