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 an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it's referring to is the player that creature was attacking at the time it became an attacking creature that combat, or the controller of the planeswalker that creature was attacking at the time it became an attacking creature that combat. If a 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 spell or 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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