Concept:Choose the singular word that correctly expresses “not one of the two.”Explanation:Sylvester and John are two people, so the sentence refers to two persons.The verb “has” is singular; therefore, the missing word must be singular.“Both” and “all” are plural, so they cannot be used.“Neither” means “not the one nor the other of two people.”It agrees with the singular verb “has” and fits the contrast introduced by “Although.”Answer:C. neither