The metre, kilogram, and second are all base units of the SI system.
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国际单位制 covers seven base units: 米 (m), 千克 (kg), 秒 (s), 安培 (A), 开尔文 (K), 摩尔 (mol), 坎德拉 (cd). Often abbreviated SI in Chinese physics textbooks too — borrowed from the French Système international d'unités.
