b) (i)   Residual Air
     During breathing out/expiration/exhalation; not all the air is removed from the lungs; the air          left inside the lungs during breathing out is residual air.
(ii)  Importance of Residual Air

• Air is left in the lungs so that the lungs do not collapse;
• air is always left inside the lungs so that more oxygen can be removed (diffuse) into the blood stream.

Candidates could not also outline the activities that can result in oxygen debt. Candidates wrote carbon monoxide in air; overcrowding, deforestation.
Expected answers are:
   (d) (i) Oxygen debt
The build up of lactic acid; during strenuous exercise is poisonous and makes the muscles ache; oxygen is needed to break down/get rid of the lactic acid; that is why athletes continue breathing heavily even after the exercise is over; the oxygen needed to get rid of the lactic acid is known as oxygen debt.                                                                                                                                       
      (ii) Activities that can result in oxygen debt
Running; boxing; swimming /any correct example of strenuous activities.