Since life began eons ago, thousands of creatures have come and gone. Some, such as the dinosaurs, became extinct due to naturally changing ecologic conditions. More recent threats to life forms are humans and their activities. Man has drained marshes, burned prairies, dammed and diverted rivers. Some of the more recent casualties of man’’s expansion have been the dodo, great auk, passenger pigeon, Irish elk, and Steller’’s sea cow. Sadly, we can no longer attribute the increasing decline in our wild animals and plant species to "natural" processes. Many species are dying out because of exploitation, habitat alteration or destruction, pollution, or the introduction of new species of plants and animals to an area. As mandated by Congress, protecting endangered species, and restoring them to the point where their existence is no longer jeopardized, is the primary objective of the U.S. Fish and Wildlife Service’’s Endangered Species Program.
Which of the following would be a likely theme for the next paragraph
A．Naturally changing ecological conditions.
B．Animals that have become extinct.
C．Achievements of the government Endangered Species Program.
D．Programs that have destroyed natural habitats.
龋洞深，叩痛（+），不松动，冷测痛并持续半分钟。
龋洞深，探酸感，叩痛（-），冷测同对照牙；
低位阻生；左下前磨牙楔状缺损中深，探硬，有酸感。余未见异常。