Which statement correctly describes the difference between hot caustic bluing and cold rust bluing?

Which statement correctly describes the difference between hot caustic bluing and cold rust bluing?

• A. Cold rust bluing is a quick process that produces a deep, dark blue finish, while hot caustic bluing is slow.
• B. Cold rust bluing uses a heated solution of sodium hydroxide salts, while hot caustic bluing uses hydrochloric and nitric acids.
• C. Cold rust bluing uses a heated solution of sodium hydroxide salts; hot caustic bluing uses hydrochloric and nitric acids.
• D. Hot caustic bluing is the most common type used by manufacturers due to its speed, involving soaking steel parts in a heated solution of sodium hydroxide salts, while cold rust bluing uses a solution of hydrochloric and nitric acids and distilled water.

Answer: (D)