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阅读理解。
David was in bed because he had been ill．Feeling better one morning, he sat up．Bright sunlight was going through the windows．“Mother!”he called out．“Come and see the tiny specks(微粒)in the air．”Mrs Lawson told David that what he could see was only dust．He asked,“Why does it come in that narrow piece of air?”His mother said,“The dust is in the air everywhere．It comes through doors and windows, but we can only see it when strong light shows it to us．It is in the air we breathe, but luckily we have hairs inside our nose．They catch dust and stop it from going down our lungs(肺)．”David's mother brought a duster and started to clean his dressing table．“See,”she said,“the dust comes down on the table, on our skin and clothes．When it is thick enough, we can see it clearly．Dust is made of soil, smoke from fires and even germs(细菌)．”“Thanks, Mum,”said David．“I was feeling better．I think I had better breathe through a handkerchief so I don't get any new germs!”
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