47.What is the main factor causing greenhouse gases?(A)Petroleum. (B)Sunlight. (C)Vapor. (D)Carbon dioxide.48.According to the second paragraph, which of the following is true?(A)Sunlight is the main cause that increases the Earth’s temperature during the past 50 years.(B)Gas using could be the main reason for the increase in global surface temperature.(C)Clouds and ice cannot help the sunlight to reflect back into spaces.(D)The rate of temperature increase has nearly doubled in the past two decades.49.Which of the following has the closest meaning to “emitted”? (A)exploited (B)strained (C)discharged (D)garnered 50.Which of the following is NOT true about this article?(A)Global warming increases Earth’s average surface temperature.(B)Without greenhouse effect, the Earth’s temperature would be much cooler. (C)Burning fossil fuels and cutting down carbon-absorbing forests cause increasing greenhouse effect. (D)Lands radiate “heat”energy due to natural greenhouse gases such as carbon dioxide.