单选题：根据下列文章,请回答题。Where Ha ve All the Frogs Gone?In the 1980s, sci

根据下列文章,请回答题。
Where Ha ve 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 a mphibians 两栖动物) are declining or ha ve become extinct. They ha ve been around for along time - over 350 million years. Why are they dying out now?
Scientists are seriously concerned about this question. First of all, a mphibians are an important source of scientific and medical knowledge. By studying a mphibians, scientists ha ve 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 a mphibians disappear.
The most serious aspect of a mphibian loss, however, goes beyond the a mphibians themselves. Scientists are beginning to think about what a mphibian decline means for the planet as A　whole. If the earth is becoming unlivable for a mphibians, is it also becoming unlivable for other kinds of animals and human beings as well?
Scientists now believe that a mphibian decline is due to several environmental factors. One of these factors is the destruction of habitat, the natural are a 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 a mphibians are no longer laying eggs. For exa mple, the arroyo toad (蟾蜍) of southern Californi a will only lay its eggs on the sandy bottom of aslow-moving strea m. There are very few strea ms left in southem California , and those strea ms are often muddy because of building projects. Not surprisingly, the arroyo toad is now in danger of extinction.
There are A　number of other factors in a mphibian 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 hea vy use of chemicals on crops has also killed off a mphibians. Another factor is that air pollution has led to increased levels of ultra violet (UV) light. This endangers a mphibians, which seem to be especially sensitive to UV light. And finally, scientists ha ve discovered A　new disease that seems to be killing many species of a mphibians in different parts of the world.
All these reasons for the disappearance of a mphibians 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 a ffect 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 beca me ill or died, the miners knew that dangerous gases wer