How can we protect the Earth？ One problem to solve is climate（气候）change． You might wonder about climate change． What is it， exactly？
Gases surround the Earth．（1） Sunlight passes down through the gases． The gases trap（储存）the sun's heat and act like blanket． Without these gases， the Earth would be very cold． The trouble comes when humans add too many gases to the atmosphere．The gases build up and trap even more heat．This causes changes in climate． The Earth becomes too warm．
（2） There's another gas causing climate change． No one talks about it as much． It is nitrous oxide（氧化亚氮）． Many call it laughing gas． Dentists may give patients nitrous oxide so they will not feel pain． The atmosphere has small amounts of nitrous oxide． But nitrous oxide can stay in the atmosphere more than 100 years which can change the climate a lot．
Nitrous oxide forms naturally． （3） Bacteria are living organisms（生物）made of single cells． Farmers use nitrogen fertilizer（氮肥）to help grow bigger crops． The bacteria in the soil take the nitrogen from the fertilizer and make more nitrous oxide with it．
So how can we help？ "Number one is to not waste food，" Wagner Riddle， a scientist from Canada， said． （4） That number comes from a U． S． university study． "There is a lot that goes into making food"， Wagner Riddle says． "So then if it's wasted， it's for nothing．"
根据短文内容，从下列选项中选出能填人文中空白处的最佳选项，选项中有一项为多余选项．
A． That's the reason for climate change．
B． It comes from bacteria（细菌）that live in the dirt．
C． Carbon dioxide（二氧化碳）is one gas that traps heat．
D． More than one﹣third of all food created in the U． S． is wasted．
E． These gases reach high into the sky and are called the atmosphere（大气层）．
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