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Lets say my opponent is running a white deck and his only permanent is a Kor Duelist. On my side I have only one permanent also, a Crowd of Cinders. He taps 3 plains, and casts O-Ring. In response, I cast go for the throat on my Crowd of Cinders. Now the only available target for O-Ring is his own Kor Duelist. Can he choose not to pick a target for his O-Ring or must he target his Kor Duelist?

Thanks