单项选择题
A.Major episodes of extinction can result from widespread environmental disturbances that affect numerous different species.
B.Certain extinction episodes selectively affect organisms with particular sets of characteristics unique to their species.
C.Some species become extinct because of accumulated gradual changes in their local environments.
D.In geologically recent times, for which there is no fossil record, human intervention has changed the pattern of extinction.
E.Species that are widely dispersed are the least likely to become extinct.
单项选择题 Currently, the number of first-time admissions of individuals diagnosed as manic-depressives to hospitals in Great Britain exceeds by nine times the number of admissions of such patients to public and private hospitals in the United States, even though the population size of the United States is many times that of Great Britain. Which of the following, if true, would be most useful to an attempt to explain the situation described above ？（）
单项选择题 A new ordinance passed by the Gorenton Council a year ago banned the sale of all nonrecyclable plastic packaging for food being sold in Gorenton. A substantial percentage of Gorenton’s plastic waste, however, is still composed of nonrecyclable plastic food packaging. Which of the following, if true, best contributes to reconciling the apparent discrepancy above ？（）
单项选择题 A tree’s age can be determined by counting the annual growth rings in its trunk. Each ring represents one year, and the ring’s thickness reveals the relative amount of rainfall that year. Archaeologists successfully used annual rings to determine the relative ages of ancient tombs at Pazyryk. Each tomb was constructed from freshly cut logs, and the tombs’ builders were constrained by tradition to use only logs from trees growing in the sacred Pazyryk Valley. Which one of the following, if true, contributes most to an explanation of the archaeologists’ success in using annual rings to establish the relative ages of the tombs at the Pazyryk site ？（）