听录音,补全段落。 Contrails are made up of ice crystals that form when aircraft __1__ emit exhaust that hits the cold air. The ice crystals reflect incoming light from the sun back into __2__, which has a cooling effect on the __3__. But the contrails also stop heat coming up from the ground from __4__ into space. Some contrails can form clouds that last for up to __5__ hours. During that time, they spread out, trapping even more heat. This process allows contrails to warm the __6__ about as much as the carbon dioxide __7__ from aircraft. But when Stettler and his team analyzed flight data they obtained of Japan airspace, they found that most contrail warming was caused by just __8__ percent of flights. And most of those flights originated in the late afternoon—because as the sun goes down, __9__ can no longer offset the warming.