阅读理解 It's true that quite a few most respected scientific  authorities have confirmed that the world is becoming hotter and hotter. There's also strong evidence that humans are contributing to the warming. Countless recent reports have proved the same thing. For instance, a 2010 summary about the climate science by the Royal Society noted that:'The global warming over the last half－century has been caused mainly by human activity.' You may not believe that humans could change the planet's climate, but the basic science is well understood. Each year, billions of tons of greenhouse gases are released into the atmosphere because of human activity. As has been known for years, these gaseshold heat that would otherwise escape to space, wrapping the planet in an invisible(看不见的) blanket. Of course, the earth's climate has always been changing due to 'natural' factors such as volcanic eruption (熔化) or changes in solar, or cycles concerning the Earth's going around the sun. According to the scientific research, however, the warming observed by now matches the pattern of warming we would expect from a build－up of greenhouse gas in the atmosphere－not the warming we would expect from other possible causes. Even if scientists did discover another reasonable explanation for the warming recorded so far, that would give birth to a difficult question. As Robert Henson puts it: 'If some newly discovered factor is to blame for the climate change, then why aren't carbon dioxide and the other greenhouse gases producing the warming that basic physics tells us they should be?' The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths－one with human influence and one without. That obviously isn't possible, and so most scientists are careful not to state human influence as an absolute certainty. 1. In most scientists' opinion, the global warming is mainly caused by________. A. solar activity B. volcanic activity C. natural factors D. human factors 2. The text is developed by________. A. giving typical examples B. following the order of space C. analyzing a theory and arguing it D. comparing and finding differences 3. The underline word 'identical' in the last paragraph probably means________. A. totally different B. exactly the same C. extremely important D. partly independent 4. Which of the following can be the best title?____ A. Are All the Scientists Really Scientific? B. Where Is Global Warming Leading Us to? C. Are Humans Definitely Causing Global Warming? D. What's Relation of Global Warming and Greenhouse Gases?