This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
[{'content': 'Structure of proteins is best studied by: \nSelect and explain. \nA. X -ray crystallography, B. Ultra centrifugation, C. Using Sangers reagent, D. Sucrose density gradient centrifugation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A Harper, 26th ed, p. 35 & 25th ed, p. 55', 'role': 'assistant'}]