In SDS-PAGE, the protein sample is first

• A. treated with a reducing agent and then with anionic detergent followed by fractionation by electrophoresis
• B. fractionated by electrophoresis then treated with an oxidizing agent followed by anionic detergent.
• C. treated with a oxidizing agent and then with anionic detergent followed by fractionation by electrophoresis
• D. none of the above