网络操作系统提供的主要网络管理功能有网络状态监控、网络存储管理和______。A．攻

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In many countries, people produce power from coal, but it is very dirty and causes acid rain. Some countries use nuclear energy to produce power.1         . To solve the energy problem, people all over the world are looking for new ways to produce power.
China is one of the first countries in the world to use biogas technology. 2         . This renewable energy is used in people’s everyday life.
3         . Now there are electric vehicles in many countries. They are very efficient and cost very little to run. 4          , there will be an increase in electricity needs if they are widely used.
In China, the best—known maglev train is the German—built one in Shanghai. The train can reach a top speed of 431 km per hour.5          . There is no wheel noise, because there are no wheels. However, maglev guide paths are much more expensive than traditional steel railways.

A．Farmers recycle straw, grass and animal waste to make biogas. B．Although electric vehicles produce no pollution. C．However, nuclear power can be very dangerous. D．It is quiet and quick. E．Electric vehicles were developed in the 1990s.