Question 15
Given triangle $ABC$, let $D$ be an inner point of the segment $BC$. Let $P$ and $Q$ be distinct inner points of the segment $AD$. Let $K = BP \cap AC$, $L = CP \cap AB$, $E = BQ \cap AC$, $F = CQ \cap AB$. Given that $KL \parallel EF$, find all possible values of the ratio $BD : DC$.