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If the Spirit or Arcane spell has {X} in the mana cost, then you use the value of {X} on the stack. For example, Shining Shoal costs {X}{W}{W}. If you choose X = 2, then Shining Shoal’s converted mana cost is 4. Shining Shoal also has an ability that says “You may exile a white card with converted mana cost X from your hand rather than pay Shining Shoal’s mana cost”; if you choose to pay the alternative cost and exile a card with converted mana cost 2, then X is 2 while Shining Shoal is on the stack and its converted mana cost is 4.