求翻译：The four swimming strokes are all performed in individual competitive events using 50-meter pools. The world records that are recognized for individual competitive swimming are: the 50 meter, 100 meter, 200 meter, 400 meter, 800 meter and 1500 meter freestyle; the 50 meter, 100 meter and 200 meter backstroke; the 50 me是什么意思？

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