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 close to the lower critical temperature and then cooled slowly