INTERROGATIVE PRONOUNS.
Three now in use.
97. The interrogative pronouns now in use are who (with the forms whose and whom), which, and what.
One obsolete.
There is an old word, whether, used formerly to mean which of two, but now obsolete. Examples from the Bible:—
Whether of them twain did the will of his father?
Whether is greater, the gold, or the temple?
From Steele (eighteenth century):—
It may be a question whether of these unfortunate persons had the greater soul.
Use of who and its forms.