After that comes another filtration - sometimes through sand or carbon.
此后到了另一个过滤池,有时是通过沙子或者碳过滤。
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Normally, silicon carbide is produced by heating sand (which is made of quartz, or silicon dioxide) in an electric furnace with carbon made from oil or coal.
The photographs actually show sand dunes coated with a thin layer of frozen carbon dioxide, or dry ice, less than 240 miles from the planet's north pole.