练习题
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。

参考翻译

1. 大多数研究者认为吸烟是患上肺癌和咽喉癌的重要原因。

　　2. 需要强调指出的是，电源仅仅是一种能让电流在电路中循环流动的装置。

　　3. 如果每一百万份水中矿物质或盐的含量少于500份，该水质则被视为高品质并适于饮用。

　　4. Covering 71 percent of the earth surface, ocean is a basic component of life support system on the earth, as well as a treasury of resource and an important adjuster of environment.