Can a player use Retreat as a reaction to save a unit that was selected to be killed by Cull the Weak?

Ruling: No, once both players pass priority after a unit is selected for Cull the Weak, the ability resolves and there is no longer an opportunity to play Retreat to save the unit.

Sequence:

• Cull the Weak is played
• A unit is selected to be killed
• Both players pass priority
• Cull the Weak resolves (unit is killed)
• No opportunity remains to play Retreat