11. Six players $A$, $B$, $C$, $D$, $E$, and $F$ are participating in a single round-robin table tennis tournament (each player plays one match against every other player). Every day, three matches are played simultaneously on three tables. It is known that on the first day, $B$ played against $D$; on the second day, $C$ played against $E$; on the third day, $D$ played against $F$; on the fourth day, $B$ played against $C$. Who will $A$ play against on the fifth day? Who will play against whom on the other two tables?
11.【Solution】On the second day, B cannot play against A, otherwise B playing against A and D playing against F would contradict the third day's D playing against F, so it should be B playing against F, A playing against D.

On the third day, B also cannot play against A, otherwise C playing against E would contradict the second day's C playing against E, it should be B playing against E (B cannot play against C, which would contradict the fourth day), A playing against C, on the fourth day B playing against C, D playing against E, so on the fifth day B playing against A, D playing against C, E playing against F.