单项选择题
In his 1979 book, The Sinking Ark, biologist Norman Myers estimated that (1) of more than 100 human-caused extinctions occur each day, and that one million species (2) lost by the century’s end. Yet there is little evidence of (3) that number of extinctions. For example, only seven species on the (4) species list have become extinct (5) the list was created in 1973.
Bio- (6) is an important value, according to many scientists. Nevertheless, the supposed (7) extinction rates bandied about are achieved by multiplying unknowns by (8) to get imponderables.
Many estimates, for instance, rely a great deal on a "species-area (9) ", which predicts that twice as many species will be found on 100 square miles (10) on ten square miles. The problem is that species are not distributed (11) , so which parts of a forest are destroyed may be as important as (12) .
(13) , says Ariel Lugo, director of the International Institute of Tropical Forestry- in Puerto Rico, "Biologists who predict high extinction rates (14) the resiliency of nature. "
One of the main causes of extinctions is (15) According to the Consultative Group on International Agricultural Research, what destroys (16) trees is not commercial logging, but "poor farmers who have no other (17) for feeding their families than slashing arid bunting a (18) of forest".
In countries that practice modern (19) agriculture, forests are in no danger. In 1920, U.S. forests covered 732 million acres. Today they cover 737 million. Forests in Europe (20) from 361 million to 482 million acres between 1950 and 1990.
A. a figure
B. a total
C. an amount
D. an average