If the inner lining of the air sacs is neither thin nor highly vascularized, then what can be inferred about the air sacs? A) They must not belong to the respiratory system. B) They cannot be derived from endoderm. C) They cannot be sites of gas exchange between air and blood. D) They must obtain nutrition from some source other than the bloodstream. E) They cannot effectively moisturize the air before it reaches the lungs.