The most popularly known blood grouping is the ABO grouping. It is named ABO and not ABC, because "O" in it refers to having :

1. A. No antigens A and B on RBCs
2. B. Other antigens besides A and B on RBCs
3. C. Overdominance of this type on the genes for A and B types
4. D. One antibody only-either anti-A and anti-B on the RBCs

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