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: Social & Preventive Medicine
Which of the following is used as reference protein in nutrition studies? 
Select and explain. 
A. Egg, B. Fish, C. Milk, D. All of the above
A: Egg
Answer: A: Egg 
Explanation: Egg protein is considered the best because of high biological value and easy digestibility. Thus egg is used as reference protein in nutrition studies. Ref: Park 22nd edition, page 564.