OG详解-OG10 文法 Q8

Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The singular verb “is” agrees in number with the singular subject “each one of a ghazal’s couplets.” While the prepositional phrase “of a ghazal’s couplets” within the subject contains a plural noun, the head of the subject (“each one”) is singular, indicating that each individual couplet (not the couplets as a group) is “thematically and logically autonomous,” or self-standing. 
Choice B is incorrect because the plural verb “were” doesn’t agree in number with the singular subject “each one of a ghazal’s couplets.” Choice C is incorrect because the plural verb “have been” doesn’t agree in number with the singular subject “each one of a ghazal’s couplets.” Choice D is incorrect because the plural verb “are” doesn’t agree in number with the singular subject “each one of a ghazal’s couplets.”