时刻测知紫外线，预防皮肤癌！
了解产品的质量，焊接和无损检测知识。
Base Knowledge For Material Quality, Visual Welding Quality, NDT, Etc.
由于暗物质不可见，天文学家们采取了间接途径来测知它的存在。
Because dark matter cannot be seen astronomers detected its presence through indirect evidence.
由于暗物质不可见，天文学家们采取了间接途径来测知它的存在。
Because dark matter cannot be seen astronomers detected its presence through indirect evidence.
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