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Classic usage
Estimated CEFR level: C2 — Proficiency
Estimated from word frequency; not an official CEFR classification.
From the prefix hydro-, from Ancient Greek ὕδωρ (húdōr, “water”). By surface analysis, hydr- (“water”) + -o-. The Canadian "electricity" sense is owing to hydroelectricity being the prevalent source of electricity in much of Canada and many electric companies having the word Hydro in their names.
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Classic usage
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