Rule 608.2g

If an effect requires information from the game (such as the number of creatures on the battlefield), the answer is determined only once, when the effect is applied. If the effect requires information from a specific object, including the source of the ability itself or a target that's become illegal, the effect uses the current information of that object if it's in the zone it was expected to be in; otherwise, the effect uses the object's last known information. See rule 112.7a. (Note that if an effect divides or distributes something, such as damage or counters, as a player chooses among some number of target creatures or players, the amount and division were determined as the spell or ability was put onto the stack rather than at this time; see rule 601.2d.) If the ability text states that an object does something, it's the object as it exists -- or as it most recently existed -- that does it, not the ability.

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