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Fill in the blanks with the given words in proper forms(字数不限)
1.As a natural resource, air 1 (find) in nature. It 2 (be) in the atmosphere. In fact, the atmosphere 3 (consist) of air, and the Earth 4 (surround) by it.
2.Recent studies have provided additional evidence (证据) that 5 (inhale) smoke from other people's cigarettes, 6 (call) passive smoke, may be harmful to a young person.… Over a period of time, a fatty substance may build up in arteries (动脉), 7 (slow) the flow of blood.When an artery 8 (lead) to the heart suddenly 9 (become) completely blocked, a heart attack may 10 (occur).
3.Scientists 11 (become) aware of the substance 12 (know) as vitamins around the year 1910. Until that time, it 13 (not understand) that certain harmful body conditions could 14 (cause) by something which 15 (not eat).
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