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Birds enjoy a relatively slow rate of extinction but a new study suggests that rate might be severely underestimated(低估). Even worse, if human actions continue, bird extinction rates could skyrocket and 12 percent of the known bird species(物种) could be dying out by the end of the century.

Presently there are 10,000 known bird species — most identified after 1850 — and an estimated 130 of those have become extinct since 1500, setting the extinction rate at about one species every four years.

But according to Stuart Pimm of Duke University, this rate fails to take into account three key points: The continual identification of extinct bird species from fossil remains; numerous "missing" species not yet declared extinct; and the fact that present extinction rates were not calculated using the proper baseline date for when the species was first described.

Taking these points into consideration, the extinction rate is closer to one bird species per year, says Pimm, the leading author of the study. And the rate could be three times as high as that if not for recent bird preservation efforts.

In previous centuries, bird extinctions took place mainly on islands as Polynesian peoples expanded into the Pacific or Europeans took over the Americas, wiping out birds along the way.

In recent years, scientists are seeing an increased number of extinctions on continents, again because of human activities. Habitat destruction, introduction of invasive species, and climate change combine to harm many bird species. Before human influence, the estimated rate of bird extinctions would have been only one species per hundred years, researchers estimate.

Bird extinction rates are slower than for most animals, mainly because humans do more harm to other species and people take special efforts to protect birds. Still, if the present trends continue, the researchers estimate that the bird extinction rate will continue to climb to as many as 10 species per year,

47. The underlined word "skyrocket" in the first paragraph means

A. fly high B. appear soon

C. increase quickly D. change regularly

48. According to Pimm, the extinction rate of birds is____________.

A. one species per hundred years

B. one species every year

C. three species per year

D. ten species every year

49. What can we infer from the fifth and sixth paragraphs?

A. Humans have started destroying bird habitat in recent years.

B. Humans are doing more harm to animals than to birds.

C. Humans have made no effort to protect birds.