A、Because they are kept away from dangers that happened before. B、Because they have learned how to overcome the particular dange

Fear and its companion pain are two of the most useful things that man and animals possess if they are used. If fire didn’t hurt when it burned, children would play with it until their hands were burned away.(19)Similarly, if pain existed but fear didn’t, a child could burn itself again and again because fear would not warn it to keep away from the fire that had burnt it before. Fear and pain are therefore two guards without which man and animals might soon die out.
     (20)We suggested that fear ought to be properly used. If, for example, you never go out of your house because of the danger of being knocked down in the street by a car, you are letting fear rule you too much.(20)The important thing is not to let fear rule you, but instead, to use fear as your servant and guide.(21)Fear will warn you of dangers; then you have to decide what action to take.
     (21)In many cases, you can take quick and successful action to avoid the danger. For example, you see a car coming straight towards you; fear warns you, you jump out of the way, and all is well. In some cases, however, you decide that there is nothing that you can do to avoid the danger. For example, you can’t prevent an airplane crashing into your house, and you may not want to live in a desert where there are no airplanes. In this case, fear has given you its warning, you have examined it and decided on your course of action, so fear of the particular danger is no longer of any use to you, and you have to try to overcome it.
19. What will probably happen if playing with fire causes pain but no fear?
20. How can fear be correctly used?
21. Why can people avoid dangers in time in many cases?