This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
The player whose turn it is. Usually if both players can do
something, the active player gets to do so first... including making
choices (which means that when the opponent chooses, he knows what choice
was made) and putting triggered abilities onto the stack (which means the
opponent's abilities will resolve first).
 
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