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If {B} was spent to cast this spell, you may have target creature get -3/-3 until end of turn. If {G} was spent to cast this spell, you may have target creature get +3/+3 until end of turn. (Do both if {B}{G} was spent.)
As you cast Cankerous Thirst, you must target a creature for the first effect and you must target a creature for the second effect. Both targets may be the same. As Cankerous Thirst resolves, you decide whether to apply either effect.
If the spell is copied, the copy will never have had mana of the stated color paid for it, no matter what colors were spent on the original spell.
The spell cares about what mana was spent to pay its total cost, not just what mana was spent to pay the hybrid part of its cost.
The spell checks on resolution to see if any mana of the stated colors was spent to pay its cost. If so, it doesn’t matter how much mana of that color was spent.
When you cast the spell, you choose its targets before you pay for it.
