【单选题】

Scientists have discovered that for the last 160,000 years, at least, there 　　has been a consistent relationship between the amount of carbon dioxide in the air 　　and the average temperature of the planet. The importance of carbon dioxide in 　　regulating theEarth’’s temperature was confirmed by scientists working in eastern(5)Antarctic

A、Drilling down into a glacier, they extracted a mile-long cylinder of ice 　　from the hole. The glacier had formed as layer upon layer of snow accumulated year 　　after year. Thus drilling into the ice was tantamount to drilling back through time. 　　The deepest sections of the core are composed of water that fell as snow 160,000 years 　　ago. Scientists in Grenoble, France, fractured portions of the core andtemperature and of atmospheric(10)measured the composition of ancient air released from bubbles in the ice. Instruments 　　were used to measure the ratio of certain isotopes in the frozen water to get an idea 　　of the prevailing atmospheric temperature at the time when that particular bit of 　　water became locked in the glacier. The result is a remarkable unbroken record of (15)levels of carbon dioxide.Almost every time the chill of an ice age descended on the planet, carbon　　dioxide levels droppeD、When the global temperature dropped 9F (5℃), carbon dioxide levels　　dropped to 190 parts per million or so. Generally, as each ice age ended and theEarth basked in a　　warm interglacial period, carbon dioxide levels were around 280 parts per million. Through the　　160,000 years of that ice(20)record, the level of carbon dioxide in the atmosphere fluctuated between 190 and 280 parts per　　million, but never rose much higher-until the industrial Revolution beginning in the eighteenth　　century and continuing today. There is indirect evidence that the link between carbon dioxide levels　　and global temperature change goes back much further than the glacial recorD、Carbon(25)dioxide levels may have been much greater than the current concentration during theCarboniferous　　perioD、360 to 285 million years ago. The period was named for aprofusion of plant life whose　　buried remains produced a large fraction of the coal deposits that am being brought to the surface　　and burned today. According to the passage, the drilling of the glacier in easternAntarctica was important because it
A、allowed scientists to experiment with new drilling techniques
B．permitted the study of surface temperatures in an ice-covered region ofEarth
C．provided insight about climate conditions in earlier periods
D．confirmed earlier findings about how glaciers are formed