Sylphia, Spirit of Air

Arrive (When this card enters the field zone.) – Choose even or odd cost. Your opponent places all of their Units with the chosen cost on the bottom of their owners’ decks in any order.

Q: When I play Sylphia, Spirit of Air, do I choose even or odds when the Arrive ability triggers or when it resolves?
A: You must choose options upon activation of the ability. You will choose even or odds when the Arrive ability triggers. (See Play Cards and Abilities 9.4.3)
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