A、All sorts of snakes in the world. B、How the skiers climb a slope on skis. C、The special movement of the side-winder. D、The dir

　　(22) We’ve now discussed how most snakes move, but there are some exceptions. One is the snake called the American side-winder. (23) From its name, you can probably guess that it moves sideways rather than in a straight line. This is because it lives in the desert where the sand slips and slides. With nothing firm to push against, the side-winder has to adapt its way of movement to the shifting sand. It pushes against the sand with the entire side of its body, and then moves sideways. Think about how a skier climbs a slope on skis. The skier places the skis at an angle of 90 degrees to the direction of movement. The skier then moves each ski by taking a step to the side up the slope. The side-winder snake moves in much the same manner. It lays its body at about 60-degree angle to the direction in which it wants to go. By doing this, the snake has more sand to push against. (24) Then it points its head in the direction it wants to go and leaps to a parallel spot. If you see the tracks left by side-winder in loose sand, you’ll see a series of paralleled lines. (25) An advantage gained from this method of moving may be that the snake’s body is kept cooler by breaking off contact with the hot sand during its leaps.
　　22.  What is the main topic of the passage?
　　23.  Why does the side-winder move the way it does?
　　24.  What do the side-winder’s tracks look like?
　　25.  What may be a special advantage of the side-winder’s way of moving?