Which is incorrect ? (C.B.S.E.2006)

• A. Presence of nonrespiratory air sacs increases efficinency of respiration in birds
• B. In insects,circulation body fluids serve to distribute oxygen to tissues
• C. Principle of counter - current flow facilitates efficient respiration in gills of fishes
• D. Residual air in lungs slightly decreases the efficiency of respiration in mammals