Read the passage below carefully and then answer the question. (from discovery education: endangered species. funk and wagnalls. 2005.) species become extinct or endangered for a number of reasons, but the primary cause is the destruction of habitat. drainage of wetlands, conversion of shrub lands to grazing lands, cutting and clearing of forests (especially in the tropics, where destruction of the rain forests has continued at an alarming rate), urbanization and suburbanization, and highway and dam construction have seriously reduced available habitats. as the various habitats become fragmented into "islands," the remaining animal populations crowd into smaller areas, causing further habitat destruction. species in these small islands lose contact with other populations of their own kind, thereby reducing their genetic variation and making them less adaptable to environmental change. these small populations are highly vulnerable to extinction; for some species, the fragmented habitats become too small to support a viable population. since the 1600s, commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. the slaughter of great whales for oil and meat, for example, has brought them to the brink of extinction; the african rhinoceroses, killed for their horn, are also critically endangered. the great auk became extinct in the 19th century because of overhunting, and the carolina parakeet perished as a species because of a combination of overhunting and habitat destruction. introduced diseases, parasites, and predators against which native flora and fauna have no defenses have also exterminated or greatly reduced some species. the accidental introduction of a blight, for example, eliminated the chestnut tree from north american hardwood forests. predator and pest control also have adverse effects. excessive control of prairie dogs, for example, has nearly eliminated one of their natural predators, the black-footed ferret. pollution is another important cause of extinctions. toxic chemicals—especially chlorinated hydrocarbons such as dichloro-diphenyl-trichloroethane (ddt) and polychlorinated biphenyls (pcbs)—have become concentrated in food webs, affecting most strongly those species at the end of the chain (see both ddt and the pcbs, for example, interfere with the calcium metabolism of birds, causing soft-shelled eggs and malformed young. pcbs also impair reproduction in some carnivorous animals. water pollution and increased water temperatures have wiped out endemic races of fish in several habitats. which sentence below best describes the main idea of this passage?