In Game 2 of The Book of Why, the causal path of interest is X rightarrow E rightarrow Y. The variables $A$, $B$, $C$, and $D$ are pretreatment variables. The only potential back-door path is X leftarrow A rightarrow B leftarrow D rightarrow E rightarrow Y. This path is already blocked because $B$ acts as a collider (A rightarrow B leftarrow D). Consequently, no variables need to be controlled to satisfy the back-door criterion. However, if confounding is accidentally introduced by controlling for the collider $B$ (or its descendant $C$), which opens the back-door path, deconfounding can still be achieved by additionally controlling for either $A$ or $D$. Traditional statistical approaches that unconditionally control for all observed variables (like $B$ and $C$) are incorrect here because adjusting for the collider $B$ opens a non-causal path, confounding the relationship between $X$ and $Y$.