Respuesta :
The store gave out samples by the dozen. - Is the correct phrase, as an adverbial phrase is a word/ phrase that modifies the/ qualifies an adjective. So the 'dozen' modifies the sentence.
The store gave out samples by the dozen. - Is the correct phrase, as an adverbial phrase is a word/ phrase that modifies the/ qualifies an adjective. So the 'dozen' modifies the sentence.