Graph Algorithms · Superchallenge problem

Six tasks $A,B,C,D,E,F$ have prerequisites: $A$ before $B$, $A$ before $C$, $B$ before $D$, $C$ before $D$, $C$ before $E$, $D$ before $F$, $E$ before $F$. These do not yet determine a unique order. What is the FEWEST additional prerequisite relations that must be added so that exactly one valid order remains?