OG详解-OG7 文法 Q1

Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The singular verb “describes” agrees in number with the singular subject “map.” 
Choice B is incorrect because the plural verb “describe” doesn’t agree in number with the singular subject “map.” Choice C is incorrect because the plural verb “have described” doesn’t agree in number with the singular subject “map.” Choice D is incorrect because the plural verb “are describing” doesn’t agree in number with the singular subject “map.”