Câu hỏi:

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

 

          Experts in climatology and other scientists are becoming extremely concerned about the changes to our climate which are taking place. Admittedly, climate changes have occurred on our planet before. For example, there have been several ice ages or glacial periods.
          These climatic changes, however, were different from the modern ones in that they occurred gradually and, as far as we know, naturally. The changes currently being monitored are said to be the result not of natural causes, but of human activity. Furthermore, the rate of change is becoming alarmingly rapid.
          The major problem is that the planet appears to be warming up. According to some experts, this warming process, known as global warming, is occurring at a rate unprecedented in the last 10,000 years. The implications for the planet are very serious. Rising global temperatures could give rise to such ecological disasters as extremely high increases in the incidence of flooding and of droughts. These in turn could have a harmful effect on agriculture.
          It is thought that this unusual warming of the Earth has been caused by so-called greenhouse gases, such as carbon dioxide, being emitted into the atmosphere by car engines and modern industrial processes, for example. Such gases not only add to the pollution of the atmosphere, but also create a greenhouse effect, by which the heat of the sun is trapped. This leads to the warming up of the planet.
          Politicians are also concerned about climate change and there are now regular summits on the subject, attended by representatives from around 180 of the world’s industrialized countries. Of these summits, the most important took place in Kyotoin Japanin 1997. There it was agreed that the most industrialized countries would try to reduce the volume of greenhouse gas emissions and were given targets for this reduction of emissions.
          It was also suggested that more forests should be planted to create so-called sinks to absorb greenhouse gases. At least part of the problem of rapid climate change has been caused by too drastic deforestation.
          Sadly, the targets are not being met. Even more sadly, global warnings about climate changes are often still being regarded as scaremongering

Greenhouse gases cause the warming up of the Earth because they _______.