Psychologists suggest that to cure cruelty

1. The writer mentions heart and circulation

1. 
   because they are the most important structures in the human body
   
2. 
   since students have great difficulty understanding their relationship
   
3. 
   in order to illustrate the extent to which students should be taught human biology
   
4. 
   as these parts of the body are frequently damaged by excessive exercise
   

2. We can infer from the passage that by training the heart

1. 
   people can expect to live much longer
   
2. 
   we become capable of doing more complicated activities
   
3. 
   the lungs' function in the partnership becomes virtually redundant
   

D) we can prevent the lungs from overworking