Rule 802.2a

Any rule, object, or effect that refers to a "defending player" refers to one specific defending player, not to all of the defending players. If the source of an ability that refers to a defending player is an attacking creature, it will usually refer to the player it's attacking or the controller of the planeswalker it's attacking. Similarly, if a spell or ability refers to both an attacking creature and a defending player, unless otherwise specified, the defending player it's referring to is the player that creature is attacking or the controller of the planeswalker it's attacking. If the spell or ability could apply to multiple attacking creatures, the appropriate defending player is individually determined for each of those attacking creatures. If there are multiple defending players that could be chosen, the controller of the ability chooses one. Example: Rob attacks Alex with Runeclaw Bear and attacks Carissa with a creature with mountainwalk. Whether the creature with mountainwalk is unblockable depends only on whether Carissa controls a Mountain.

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