Rule 608.2d

If an effect of a spell or ability offers any choices other than choices already made as part of casting the spell, activating the ability, or otherwise putting the spell or ability on the stack, the player announces these while applying the effect. The player can't choose an option that's illegal or impossible, with the exception that having an empty library doesn't make drawing a card an impossible action (see rule 119.3). Example: A spell's instruction reads, "You may sacrifice a creature. If you don't, you lose 4 life." A player who controls no creatures can't choose the sacrifice option.

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