Psychologists suggest that to cure cruelty

1. According to the passage, over 500 years before the first balloon flight

C) boomerangs were the only means of air travel for men

1. 
   men didn't even know how to fire arrows
   
2. 
   Leonardo da Vinci studied the flight of birds and drew flying machines
   

D) Knowledgeable men believed successful flights would be achieved one day

2. It is clear from the passage that Leonardo da Vinci

1. 
   had considered the possibility of air travel in some depth
   
2. 
   concentrated more on science than on art
   
3. 
   had ignored the existence of air pressure
   
4. 
   had made detailed studies of how a boomerang returns to its thrower