You will be given a definition of a task first, then some input of the task.
In this task, you're given an article, a question which often contains a blank and four options (associated with "A", "B", "C", "D"). Your task is to find the correct answer (from the given options) for the question from the given article and return one of the options from "A", "B", "C", and "D". Do not generate anything else apart from one of the following characters: "A", "B", "C", "D". There is only one correct answer for each question.

Article: Every year there are changes in climate in different parts of the world.Some of these changes are due to natural causes.However,some climate changes are caused by air pollution and these changes may increase. If the pollution affects the level of carbon dioxide in the atmosphere,the results are likely to be serious.There is only a small part of carbon dioxide in the atmosphere.But it has an important function in keeping the balance between radiation from the sun entering the atmosphere and radiation leaving the Earth.Some of the radiation is absorbed by the Earth and some is radiated back into the atmosphere.The carbon dioxide in the atmosphere prevents some of the radiation from leaving the atmosphere.Thus the heat remains in the atmosphere and carbon dioxide helps to prevent the temperature of the Earth from falling. If the proportion of carbon dioxide in the atmosphere is increased as a result of air pollution,the temperature of the atmosphere may rise.This might cause the ice in the North and South to melt.If this happened,the sea level could rise and parts of the Earth would be flooded.This is not quite likely but the possibility exists. There is also a fairly strong possibility that the dust level in the atmosphere will rise as a result of industrial pollution.This dust pollution will reflect sunlight back onto space.If this happens,less sunlight will reach the Earth and the temperature will fall. Another danger comes from the destruction of the Earth's vegetation such as the forest of Brazil,which is being cleared away for farmland and cities.Trees use carbon dioxide and their destruction may upset the balance of carbon dioxide in the atmosphere. 
 Question: The main purpose of the passage is to tell us that  _  . 
 Options: (A) the climate of the earth is always changing (B) air pollution has greatly affected the climate of the world (C) carbon dioxide has something to do with climate changes (D) the climate of the world is subjected to dangers from air pollution
Output:
D