Directions: Complete the following passage by using the words in the box. Each w

Directions: Complete the following passage by using the words in the box. Each word can only be used once. Note that there is one word more than you need.

More recent research has shown that many kinds of amphibians(两栖动物) are declining or have become extinct. Amphibians are animals, such as frogs, that live partly in water and partly on land. And they have been around for over 350 million years. They have   41   three mass extinctions, including the extinction of the dinosaurs. Why are they dying out now?

The most serious aspect of amphibian loss, however, goes beyond the amphibians themselves. Scientists are beginning to think about what amphibian   42   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   43   of habitat, the natural area where an animal lives. Amphibians are very   44   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.

There are a number of other factors in amphibian decline. Pollution is one of them. In many industrial areas, air pollution has   45   the rain, which then falls on ponds and kills the frogs that live there. In farming areas, the heavy use of   46   on crops has also killed off amphibians. Another factor is that air pollution has led to    47   levels of ultraviolet （UV） light. This endangers amphibians, which seem to be   48   sensitive to UV light.

All these reasons for the disappearance of amphibians are also good reasons for more general   49   . These factors affect human beings, too.

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