Q:If you were asked what your body is made of, probably you would say that it is made of flesh and bones and covered with skin. You also know something about the blood which flows through it, and that there is a very important part which we call the brain. But the flesh and the bones make up the largest part of the body, and there is a good deal to be learned about them. You know the general appearance of both flesh and bone from the parts of animals which you see in the butcher's shop. Bone is hard, and of a white color; indeed, it looks more like a piece of wood or stone than a part of a living animal. But bone is a real part of the living body, and it grows and is nourished just as the body is. If a man breaks the bone of his arm or his leg, it is firmly bound up, to prevent the broken parts from moving out of the proper places, and in time then bone grows together again and becomes quite strong. The bones are fastened together by various kinds of joints . They form the framework of the body, and give it strength to retain its proper shape. This framework is called the skeleton . Some animals, such as worms and slugs, have no skeleton; and others, such as shellfish, have a kind of hard covering or skeleton outside. The most important part of the skeleton is the backbone. It is so important that naturalists divide all animals into two classes,--- those which have a backbone and those which have none. All the higher animals, including man, have a backbone, or vertebral column as it is called. They are therefore called vertebrate animals. The others are called invertebrate animals. Why should the bones bound up firmly if they were broken? A) To make the bones strong B) To protect our arms and legs C) To fasten the joints. D) To prevent the broken parts from moving out of the proper places.
A:
D