This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
All of the following are techniques of protein purification and separation, EXCEPT: 
Select and explain. 
A. Chromatography, B. Centrifugation, C. Electrophoresis, D. Densitometry
D: Densitometry
Answer: D: Densitometry 
Explanation: Purified proteins can be obtained by several methods: gel-filtration chromatography, ion-exchange chromatography, immunoaffinity chromatography, immunoprecipitation, and salt precipitation. New methods are, micropreparative electrophoresis, and centrifugation. Ref: Harper's Illustrated Biochemistry 27th Edition, Page 21-24; The Protein Protocols Handbook edited by John M. Walker, 1996, Page 215.