【Thereisaseriousproblemtoourearth．That'sglobalwarming．Nodoubtthetonsofcarbondioxidebeingthrownupintotheairaredangerouslywarmingtheearth．Itisclearthataveragetemperaturehasrisenduringthelastcentury．Noteveryo】

　　There is a serious problem to our earth．That's global warming．No doubt the tons of carbon dioxide being thrown up into the air are dangerously warming the earth．It is clear that average temperature has risen during the last century．Not everyone can believe that carbon dioxide is responsible．A new study shows us it is not all the fault of carbon dioxide．

　　Some scientists studied the sun and found that when the sun becomes more active，the earth warms up．When the sun quiets，as it did in the 1950s and 1960s，temperature takes a dip （下降）．So some scientists said the sun's changeable moods may be more important than carbon dioxide in warming the earth．You might think that"global warming"means nothing more than a rise in the world's temperature．But rising sea levels caused by it have affected our environment and living standard in many ways．

　　We can't change the sun's moods．So in order not to make the earth warming up，what we can do is to control the amount of giving off carbon dioxide．Studies have shown that humanrelated emissions （排放物） of carbon into the atmosphere is causing，and will in the future cause seriously global warming．Many countries have already made some laws to stop sending carbon dioxide into the air directly．As an ordinary person，we use fewer cars when we go out．We can ride a bike or walk．It is not only good for environment，but also good for our health．

　　Are you ready？It is the time that we do something for the global warming．Let us go for it．

　　21．What makes the earth become warmer and warmer？___

　　A．Carbon dioxide

　　B．The sun

　　C．The air

　　D．A and B

　　22．The scientists also said that the sun's changeable moods may be___than carbondioxide in warming the earth．

　　A．more serious 　

　　B．more difficult

　　C．more important

　　D．more terrible

　　23．What should we do in order not to make the global warming？___

　　A．We should control sending carbon