题干

舞台上喷洒干冰(固态二氧化碳)可以产生白雾,给人若隐若现的视觉效果.请简述白雾的产生原因.

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答:在舞台上洒干冰(固态的二氧化碳),可造成烟雾缭绕的景象,在此变化过程中,干冰迅速升华,从周围空气中吸收大量的热,使空气温度急剧下降,空气中的水蒸气遇冷凝结成小水珠,悬浮在空气中,形成白雾,人以若隐若现的视觉效果.

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    For the first time in the history of the Beijing, Shanghai and Hangzhou marathons, fourteen blind runners competed in the Nov 6 Hangzhou Marathon this year with the assistance of personal guides. Each runner was supported by 4 or 5 volunteers. Among them was Yan Wei, from a running group called “Running in the Dark”, which specially provides training for blind runners.

    During trainings, blind runners are each accompanied by three volunteers. One volunteer is responsible for removing any obstacles(障碍物)on the road and controlling the pace. The second volunteer acts as a lookout for other runners or vehicles on the path while the third, who is attached to the blind runner by a safety rope, takes note of the trainee's running gait and pace.

    Zhu Peihua, the leader of the group's Shanghai branch, first developed a passion for running last year when he took part in a guided running activity. Zhu said that he was so fascinated with the sport that he even bought a treadmill(跑步机)so that he could train at home. “Running gives me the chance to be outside and take in the fresh air. The activity has also made me more willing to talk to people. It's simply good for health, ” said Zhu.

    The popularity of guided running activities for blind people has been growing since last year. Cheng Yi, a volunteer at Running in the Dark's Hangzhou branch, said that both the numbers of blind runners and volunteers to become assistants to them are rising continually. However, he added that volunteers face a more physically demanding task compared to a regular runner. Some of the other challenges involve being aware of the verbal(口头的) instructions one gives.

     “At the beginning, I kept saying ‘here' and ‘there' instead of ‘left' or 'right' which is much more specific and relevant to the runner. A blind runner once collided with another runner because of my mistake. Luckily, no one was hurt,” said Cheng. Through everything, according to Cheng, he and many of the blind runners he has guided become close friends with shared topics and increased understandings.