If this Digimon has both level 3 and level 4 Digimon cards in its digivolution cards, can I choose to play just 1 of them with this card's [When Attacking] effect?
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No. If you choose to play cards with the [When Attacking] effect, you must play as many cards as possible?so if cards of both levels are in the digivolution cards, you can't play just 1 of them.
Source: BT10-027 Regalecusmon