Where Have All the Frogs Gone? In the 1980s, scientists around the world began to notice something strange： Frogs were disappearing. More recent research has shown that many kinds of amphibians （两栖动物）are declining or have become extinct. They have been around for a long time — over350 million years. Why are they dying out now? Scientists are seriously concerned about this question. First of all, amphibians are an important source of scientific and medical knowledge. By studying amphibians, scientists have learned about new substances that could be very useful for treating human diseases.Further research could lead to many more discoveries, but that will be impossible if the amphibians disappear. The most serious aspect of amphibian loss, however, goes beyond the amphibians themselves. Scientists are beginning to think about what amphibian decline means for the planet as a whole. If the earth is becoming unlivable for amphibians, is it also becoming unlivable for other kinds of animals and human beings as well? Scientists now believe that amphibian decline is due to several environmental factors. One of these factors is the destruction of habitat, the natural area where an animal lives. Amphibians are very sensitive to changes in their habitat. If they cannot find the right conditions, they will not lay their eggs. These days, as wild areas are covered with houses, roads, farms, or factories, many kinds of amphibians are no longer laying eggs. For example, the arroyo toad （蟾蜍）of southern California will only lay its eggs on the sandy bottom of a slow-moving stream. There are very few streams left in southern California, and those streams are often muddy because of building projects. Notsurprisingly, the arroyo toad is now in danger of extinction. There are a number of other factors in amphibian decline. Pollution is one of them. In many industrial areas, air pollution has poisoned the rain, which then falls on ponds and kills the frogs and toads that live there. In farming areas, the heavy use of chemicals on crops has also killed off amphibians. Another factor is that air pollution has led to increased levels of ultraviolet (UV) light. This endangers amphibians, which seem to be especially sensitive to UV light. And finally, scientists have discovered a new disease that seems to be killing many species of amphibians in different parts of the world. All these reasons for the disappearance of amphibians are also good reasons for more general concern. The destruction of land, the pollution of the air and the water, the changes in our atmosphere, the spread of diseases — these factors affect human beings, too. Amphibians are especially sensitive to environmental change. Perhaps they are like the canary（金丝雀） bird that coal miners once used to take down into the mines to detect poisonous gases. When the canary became ill or died, the miners knew that dangerous gases were near and their own lives were in danger.Scientists think that the decline of amphibians could A cause environmental change.B cause a decline in other kinds of animals.C be a warning signal for human beings.D be a good sign for human beings.