单项选择题 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.

单项选择题 Of the various schemes that have been proposed to model how gene transcription might work in the living body, those involving actual physical contact between enhancers and genes remain （）, owing to an absence of direct evidence.

单项选择题 An excellent philatelist can detect a forged （） with a single glance, for a true specimen bears markings and characteristics, without exception, that the（）invariably lacks.