Given the sets:

BLOODTYPES $=\{$A$,$ B$,$ AB$,$ O$\}$

TRANSFUSIONS $=\{$Transfusion$1,$ Transfusion$2,$ Transfusion$3,$ Transfusion$4\}$

The list of blood types covered by each TRANSFUSIONS element:

Transfusion$1=\{$A$,$ AB$\}$

Transfusion$2=\{$B$,$ AB$\}$

Transfusion$3=\{$AB$\}$

Transfusion$4=\{$A$,$ B$,$ AB$,$ O$\}$

What is the set covering of Transfusion types to enable transfusions for the complete blood type set?