维生素的生产方法包括化学合成、半合成、生物发酵和用生物材料直接提取。
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电压与电流成正比例关系，比例系数称为元件的。（填中文）
In this part, you are going to hear five sentences. Listen and write down what you have heard in the given area.[音频]
[音频]Listen to the recording and write down what you have heard in the answer area.
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