A、Birds will not have enough food to survive the winter. B、There are too many birds living in the same place. C、Birds reiving on

(19)Attracting and feeding wild birds are entertaining activities that have long been enjoyed by people all over the world. Feeding birds has become so popular that prepared feed mixtures are readily a-vailable. We feed birds for many reasons. Many pleasant hours can come from watching birds. A hobby often develops into a serious study of their habits. Accurate identification of birds is usually the first goal. But observations that an amateur birdwatcher can make are really limitless. There is, however,(20)responsibility associated with bird feeding, including a disease hazard. Attracting numbers of birds continually to the same spot can be harmful to them, particularly species that pick food from the ground contaminated by the droppings of other birds. In winter feeding efforts are most satisfying to people and are of greatest benefit to birds. During this time when fewer natural foods are available and air temperatures are lower, extra feeding can keep a bird warm and well. Once begun. feeding should never stop during these lean months. If you start a local increase of birds, be prepared to do what may be required to eliminate hazards to those you want to befriend.(21)A constant supply of food should be given until the cold is over and spring has come. If feeding is stopped during severe weather, birds used to relying upon the feeders must starve.
Questions 19 to 21 are based on the passage you have just heard.
19. What does the speaker say about feeding birds?
20. Why can feeding birds sometimes be harmful to them?
21. Why should feeding never stop once begun during lean months of winter?