21.(强调句型)It was a scientist called Charles Keeling, who made accurate measurements of the amount of carbon dioxide in the atmosphere from 1957 to 1997.(是一位名叫查尔斯·基林的科学家曾经把1957-1997年期间大气层中二氧化碳的含量作了精确的统计。)

　　22.(宾语从句)They also agree that it is the burning of more and more fossil fuels that has resulted in this increase in carbon dioxide.(他们还赞同下述观点，正是燃烧越来越多的化石燃料导致了二氧化碳的增加。)

　　23.(On the one hand...On the other hand...一方面……另一方面……)On the other hand, there are those, like George Hambley, who are opposed to this view, believe that we should not worry about high levels of carbondioxide in the air.(另一方面，还有一些人，像科学家乔治·汉布利，反对上面的观点，他们认为我们不必担心空气中会有高含量的二氧化碳。)

　　24.(条件状语从句)It's OK to leave an electrical appliance on so long as you are using it — if not, turn it off!(在用电器设备时你可以让它开着，如果不用就把它关掉!)

　　25.(现在分词完成时作状语)Having collected and evaluated the information, I help other scientists to predict where lava from the volcano will flow next and how fast.(收集和评估了这些信息之后，我就帮助其他科学家一起预测下次火山熔岩将往何处流去，流速是多少。)

　　26.(be about to do...when...“正要……这时……”)I was about to go back to sleep when suddenly my bedroom became as bright as day.(我刚要再睡,突然我的卧室亮如白昼。)

　　27.(现在分词完成时作状语)Having studied volcanoes now for many years, I am still amazed at their beauty as well as their potential to cause great damage.(虽然我从事火山研究很多年了,但是我对火山的壮丽景色以及它那潜在的巨大破坏力至今仍然感到惊愕不已。)

　　28.(and连接并列结构)The height of the land varies from 700 metres above sea level to over 2,000 metres and is home to a great diversity of rare plants and animals.(这里地面的高度从海拔700米到2000多米不等,是多种多样稀有动植物的生长地。)

　　29.(not only...but (also)... “不仅……而且……”)When you arrive you are rewarded not only with the sight of its clear waters, but also by the view of the other sixteen mountain peaks that surround Tianchi.(你一到达山顶就会得到回报一一你不仅可以看到天池那清澈如镜的湖水，而且还可以看到天池四周的16座山峰。)

　　30.(it is said that...“据说……”是固定句式)It is said that this boy, who had a great gift for language and persuasion, is the father of the Manchu people.(据说这个男孩就是满族人的祖先，具有语言天赋和很强的说。)