| 21. | Water is a compound of hydrogen and oxygen . |
| 水是氢氧化合物。
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| 22. | A plant drinks in oxygen from the atmosphere. |
| 植物从空气中吸取氧气。
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| 23. | It adapts itself to water with low oxygen content. |
| 它能适应含氧量少地水。
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| 24. | The union of hydrogen and oxygen forms water. |
| 氢和氧结合生成水。
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| 25. | Oxygen unites with hydrogen to form water. |
| 氧和氢结合生成水。
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| 26. | Oxygen exists free in air. |
| 氧游离于空气中。
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| 27. | Hydrogen and oxygen combine to form water. |
| 氧和氢化合构成水。
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| 28. | Water contains hydrogen and oxygen . |
| 水含有氢和氧。
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| 29. | While the five babies were still quietly sleeping in oxygen tents in hospital nursery, their parents were paying the price for fame. |
| 眼下，五胞胎还静静地躺在医院婴儿室的氧气帐里，他们的父母却为这名声付出了代价，
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| 30. | It is bacteria that have generated the oxygen we breathe on earth. |
| 正是细菌生产出我们在地球上呼吸的氧气。
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