Who has four legs but doesn’t walk?
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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.
Dùng phương pháp đánh giá em nhé.
Nếu p = 2 ta có: 2 + 2 = 4 (loại)
Nếu p = 3 ta có: 2 + 3 = 5; 3 + 7 = 7 (thỏa mãn)
Nếu p > 3 ta có
P = 3k + 1 hoặc p = 3k + 2
Nếu p = 3k + 1 thì p + 2 = 3k + 1 + 2 = 3k + 3 ⋮ 3 (là hợp số loại)
Nếu p = 3k + 2 thì p + 4 = 3k + 2 + 4 = 3k + 6 ⋮ 3 (là hợp số loại)
Vậy p = 3 thì p + 2 và p + 4 là số nguyên tố
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The chair, the table, the bed,...