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In a hardening process, the hypo-eutectoid steel is

A. Heated from 30°C to 50°C above the upper critical temperature and then cooled in still air

B. Heated from 30°C to 50°C above the upper critical temperature and then cooled suddenly in a suitable cooling medium

C. Heated from 30°C to 50°C above the upper critical temperature and then cooled slowly in the furnace

D. Heated below or closes to the lower critical temperature and then cooled slowly

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