Beavers  are a very unique species in the world of nature and wildlife----they are architects. Their dams are fine examples of engineering. A pair of beavers will build a dam, using branches, mud and stones, across a river. The water held back by the dam flows over the bank on either side of the river, flooding the nearby ground and forming a pond. At some point in the pond the beavers then build their home, which is called a lodge. This consists of a cone-shaped  pipe of branches and sticks of two to six feet in length held together with mud and stones, the top of which projects above the waterline. It serves as a shelter from the bad weather, a safety from enemies and a base for food supplies to be drawn upon in winter. From an engineering point of view the lodge could hardly be improved. Not only does it contain a central room just above water level, which is accessible only through underwater tunnels, but it also has "walls", one or more escape tunnels and an air hole at the top, which controls the temperature inside and gives air-conditioning. It is altogether a clever piece of construction, with all modern conveniences. It is, in fact, better protected against the effects of flooding than many human houses. Trees are essential to beavers. They eat the bark on the upper branches. But they must first _ the trees, using their four front teeth. With these sharp tools, it takes only a few minutes to cut down a tree. The engineering skill of beavers is to a large extent a result of their ability to use their front paws as hands. A female will carry her young held under her chin  with her front paws, walking on her hind legs. A similar method is used by all beavers when transporting stones or mud, although they also carry such materials on their broad flat trails. The fore-paws are also used for digging and for dragging heavier pieces of wood. This text is mainly about   _  .
the beaver's lodge the beaver's engineering skills the beaver's dam the beaver's intelligence