题目内容：
Directions: Each sentence below has one or two blanks, each blank indicating that something has been omitted. Beneath the sentence are five lettered or sets of words. Choose the word or set of words for each blank that best fits the meaning of the sentence as a whole.Previous research has established the lupine wolf as the __________of the modern canine, but when and where humans first domesticated the former into the latter remains unclear.
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