A 24-year-old medical student is having some difficulty in understanding the role that antibodies play in the destruction of cells due to complement lysis. The professor decides that "seeing is believing" and so sends the student to the laboratory. The student is given two flasks. One flask contains antibodies isolated from an individual whose ABO phenotype is A. In the other flask there are red blood cells from an individual whose phenotype is unknown. The student is directed to put the red blood cells into the dish containing agar, and to add the isolated antibodies.After mixed for 3mins, he found that red blood cells agglutinated. The phenotype of the individual from whom the red blood cells were isolated could have been ( )