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1. Cigarette smoking is believed by most research workers in this field to be an important factor in the development of cancer of the lungs and cancer of the throat.
2. It is to be emphasized that a source of electricity current is simply a device for causing electricity to move around a circuit.
3. Water is considered to be of high quality and drinkable if it has no more than 500 parts minerals, or salts, per one million parts of water.
4. 海洋覆盖了地球表面的71%，是全球生命支持系统的一个基本组成部分，也是资源的宝库，环境的重要调节器。
