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三相异步电机功率流程是什么？
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通信系统常用的复用技术有（）。
当浮头式换热器的传热面积和管子数目一定时，增加（）会增大流体在每一个管程内的流速。
矿用汽车后悬挂缸的作用是用来（）。
Exports are either raw materials or manufactured goods. Raw materials are products of the land， such as cotton， timber or rubber. Some raw materials， such as iron ore， come from mines. These raw materials are often exported by the countries that produce them to other countries where they are made into manufactured goods. Some countries produce food for export， for example， meat， sugar， or cereals such as wheat and maize. These countries are agricultural countries. An agricultural country needs fertile land and a good climate. A cold， dry climate is not suitable for agriculture. A country which produces manufactured goods is known as an industrialized country. An industrialized country cannot always produce enough food for its own needs. In this case， it does not export foodstuffs. Instead it has to import them. It relies on exports of manufactured products and pays for imports with the money it earns from the exported goods. From the first sentence of the passage we can know that there are（）kinds of exports.
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