The division of mammalian lungs into a very large number of tiny alveoli around alveolar ducts opening into bronchioles is:

• A
  An inefficient system of ventilation of alveoli though with very little residual air
• B
  An inefficient system of ventilating the alveoli resulting in very high percentage of residual air in the lungs
• C
  $A$ very efficient system of ventilating the alveoli with no residual air
• D
  An efficient system of ventilation of alveoli with little or no residual air