Game 5 is just Game 4 with a little extra wrinkle. Now a second back-door path X ← B ← C → Y needs to be closed. If we close this path by controlling for B, then we open up the M-shaped path X ← A → B ← C → Y. To close that path, we must control for A or C as well. However, notice that we could just control for C alone; that would close the path X ← B ← C → Y and not affect the other path.
