There are four possible combinations of blood types namely: Type A (only antigen A), Type B (only antigen B), Type AB (both antigens A and B), and Type O (neither antigens A nor B). This is referred to as the ABO blood typing system. In addition, red blood cells have a Rhesus factor or Rh, which is either present or absent. If the Rh factor is present, the cells are referred to as Rh positive.
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Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace.n-route air traffic controllers work in facilities called Air Traffic Control Centers, each of which is commonly referred to as a center. The United States uses the equivalent term Air Route Traffic Control Center (ARTCC).