3 Answers

Which witch?

Which? :http://www.which.net/

interrogativepronoun& determiner
asking for information specifying one or more people or things from a definite set: [ as pronoun ] : which are the best varieties of grapes for long keeping? | which of the suspects murdered him? | [ as determiner ] : which way is the wind blowing?
relativepronoun& determiner
used referring to something previously mentioned when introducing a clause giving further information: [ as pronoun ] : a conference in Vienna, which ended on Friday | [ after prep. ] : it was a crisis for which he was totally unprepared | [ as determiner, after prep. ] : your claim ought to succeed, in which case the damages will be substantial.
PHRASES
which is which used when two or more people or things are difficult to distinguish from each other: there is no confusion as to which is which.
ORIGIN Old English hwilc, from the Germanic bases of who and alike.
usage: In US English, it is usually recommended that which be employed only for nonrestrictive (or nonessential) clauses: the horse, which is in the paddock, is six years old (the which clause contains a nonessential fact, noted in passing; the horse would be six years old wherever it was). A that clause is restrictive (or essential), as it identifies a particular thing: the horse that is in the paddock is six years old (not any horse, but the one in the paddock). See also usage at restrictive and that.