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: Physiology
In blood bilirubin is with 
Select and explain. 
A. Protein, B. Steroid, C. Vitamin, D. Carbohydrates
A: Protein
Answer: A: Protein 
Explanation: Bilirubin is bound to protein for transportation in blood.