21. Why do people usually sing ?
A. They like birds. B. They feel happy.
C. They want to tell a story . D. They like studying music.
22. Which birds sing the most beautiful songs ?
A. Birds in a good temper. B. Cock birds.
C. Hen birds. D. Female birds which attract male birds.
23. What warnings does a bird sometimes sing ?
A. A warning to keep away. B. A warning to come quickly.
C. A warning about the approach of people D. A warning to stop singing.
24. What do most birds usually do if they cannot sing well ?
A. warn other birds to go away. B. give their information in another way.
C. find a wife. D. fly high in the sky.
25. What is one bird's unusual way of attracting a hen bird ?
A. It dances. B. It spreads out its tail.
C. It searches for a wife. D. It uses shells and flowers to make a garden.