Ở động vật chưa có cơ quan tiêu hoá, thức ăn được tiêu hoá như thế nào?

A. Tiêu hóa ngoại bào

B. Tiêu hoá nội bào

C. Tiêu hóa ngoại bào và tiêu hoá nội bào

D. Một số tiêu hoá nội bào, còn lại tiêu hoá ngoại bào

Read the passage carefully and choose the correct answer.

 People usually sing because they like music or because they feel happy. They express their happiness by singing. When a bird sings however, its song usually means much more than that the bird is happy. Birds have many reasons for singing. They sing to give information. Their songs are their language.

The most beautiful songs are sung by male (cock) birds. They sing when they want to attract a female (hen) bird. It is their way of saying that they are looking for a wife.

Birds also sing to tell other birds to keep away. To a bird, his tree or even a branch of tree, is his home. He does not want strangers to come near him, so he sings to warn them.

If a bird cannot sing well, he usually has some other means of giving important information. Some birds dance, spread out their tails or make other signs. One bird has a most unusual way of finding a wife. It builds a small garden of shells and flowers.

 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.