Rule 117.3c

Activating mana abilities is not mandatory, even if paying a cost is. Example: A player controls Thorn of Amethyst, which says "Noncreature spells cost {1} more to cast." Another player removes the last time counter from a suspended sorcery card. That player must cast that spell if able, but doing so costs {1}. The player is forced to spend {1} if enough mana is in his or her mana pool, but the player isn't forced to activate a mana ability to produce that {1}. If he or she doesn't, the card simply remains exiled.

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