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: Pathology
Blood groups A1 and A2 differ in - 
Select and explain. 
A. Chemical structure of antigen, B. Spatial configuration of antigen, C. Molecular groups, D. Ionic charges on surface
A: Chemical structure of antigen
Answer: A: Chemical structure of antigen 
Explanation: A1 and A2 blood groups have different glycolipid antigens on erythrocyte membrane.