While this suggests their future might not be so bleak, it does not ensure that they will survive the melting occurring in the polar regions today.An international team of researchers has just completed the most comprehensive analysis yet of the polar bear genome (基因组). The team examined DNA from 23 living polar bears and a 110,000-year-old polar bear jawbone. Aided by comparisons with the genomes of brown and black bears, they found that polar bears first emerged as a separate species between four and five million years ago. Previous studies had suggested the species didn't diverge (分化) from brown bears until much later — perhaps just 600,000 years ago.The analysis also showed that polar and brown bears have sometimes interbred (杂交) since their initial divergence and that populations of polar bears have been changing over the past million years in accordance with the climate. Polar bear numbers dropped during warm periods but soared again with prolonged ice ages — most noticeably during a cooling period between 800,000 and 600,000 years ago. The population looks set to fall again as melting pack ice forces polar bears back to the land-based habitats (栖息地) of brown bears, where interbreeding has recently been observed. Should extensive interbreeding occur, both polar and brown bear populations could decline with the emergence of a new hybrid species.The team says that the analysis reveals that polar bear DNA has lost diversity as the population has decreased, drifted apart, and become genetically isolated, suggesting today's bears have less adaptability to environmental change, habitat loss, pollution, and diseases they now face.What does the author say about the future of polar bears?()A They will not survive due to the melting occurring in the polar regions today.B We cannot be sure whether they will survive.C They will certainly survive after patrolling the planet's icy regions for millions of years.D We can help them to ride out the global warming this time again.

Polar bears have patrolled the planet's icy regions for millions of years longer than previously thought — riding out several episodes of global warming in that time. While this suggests their future might not be so bleak, it does not ensure that they will survive the melting occurring in the polar regions today. An international team of researchers has just completed the most comprehensive analysis yet of the polar bear genome (基因组). The team examined DNA from 23 living polar bears and a 110,000-year-old polar bear jawbone. Aided by comparisons with the genomes of brown and black bears, they found that polar bears first emerged as a separate species between four and five million years ago. Previous studies had suggested the species didn't diverge (分化) from brown bears until much later — perhaps just 600,000 years ago. The analysis also showed that polar and brown bears have sometimes interbred (杂交) since their initial divergence and that populations of polar bears have been changing over the past million years in accordance with the climate. Polar bear numbers dropped during warm periods but soared again with prolonged ice ages — most noticeably during a cooling period between 800,000 and 600,000 years ago. The population looks set to fall again as melting pack ice forces polar bears back to the land-based habitats (栖息地) of brown bears, where interbreeding has recently been observed. Should extensive interbreeding occur, both polar and brown bear populations could decline with the emergence of a new hybrid species. The team says that the analysis reveals that polar bear DNA has lost diversity as the population has decreased, drifted apart, and become genetically isolated, suggesting today's bears have less adaptability to environmental change, habitat loss, pollution, and diseases they now face. What does the author say about the future of polar bears?() A They will not survive due to the melting occurring in the polar regions today. B We cannot be sure whether they will survive. C They will certainly survive after patrolling the planet's icy regions for millions of years. D We can help them to ride out the global warming this time again.