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Well, because reaching $A$ before $B$ could happen either by $X_1\ge x_1$ and $X_2\ge x_2$, or by $X_1\ge y_1$ and $X_2\ge y_2$ for some other values $y_1$, $y_2$... so either by $A$ and $B$, or by $C$ and $D$. Or, in a myriad of other similar ways.
