This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
Which purine base contains an amino group at carbon 6? 
Select and explain. 
A. Guanine, B. Uracil, C. Adenine, D. Cytosine
C: Adenine
Answer: C: Adenine 
Explanation: Reference: Harpers illustrated biochemistry 30th edition