单选题 .  ①Scientific consensus is that humans first began to have a warming effect on Earth's climate within the past century, after coal-burning factories, power plants, and motor vehicles began releasing carbon dioxide (CO2) and other greenhouse gases in significant quantities into the air. ②However, evidence suggests that human agricultural activities may have had such an effect much earlier: concentrations of CO2 started rising about 8000 years ago, even though natural trends indicate they should have been dropping; methane levels rose similarly about 3,000 years later. ③Without these rises, however, current temperatures in northern parts of North America and Europe would be cooler by three to four degrees Celsius—enough to inhibit agriculture—and an ice age would probably have begun several thousand years ago in northeastern Canada.45.  Which of the following best describes the function of the highlighted sentence? ______