Passage Four The frog is an amphibian, which means "double living" or both water and land living. That is, it is able to get oxygen from water at one stage in its life and from the air at another stage. Perhaps you have heard the term amphibian used for military vehicles which can travel on both land and water. Toads and salamanders, as well as frogs, are amphibians. In the development of animals over the world’s history, amphibians were the first vertebrates to live at least part time on land. As a tadpole, the frog spends the beginning of its life living in the water. The tadpole breathes by means of tiny gills. When the lungs begin to develop, the gills disappear. By the time they are adults, all frogs have lungs for breathing while on land. In this way, the animal becomes essentially a land-adapted animal. In addition to having lungs, frogs and salamanders have a moist skin through which oxygen and carbon dioxide are exchanged. This moist skin is very important in the winter weather when amphibians hibernate in the mud at the bottom of ponds and lakes. They get what little oxygen they need through the skin. Like fish, amphibians are cold-blooded animals. This means that their body temperature rises and falls with that of their surroundings. Based on paragraph 4, we conclude that （）.
A．frogs cannot move about in snowy conditions
B．frogs are cold-blooded animals
C．the body temperature of frogs rises during the winter time
D．the surroundings are too dangerous for frogs to remain on land