OG详解-OG9 文法 Q8

Choice B is the best answer. The convention being tested is subject-verb agreement. The singular verb “is” agrees in number with the singular subject “the shape.”
Choice A is incorrect because the plural verb “are” doesn’t agree in number with the singular subject “the shape.” Choice C is incorrect because the plural verb “were” doesn’t agree in number with the singular subject “the shape.” Choice D is incorrect because the plural verb “have been” doesn’t agree in number with the singular subject “the shape.”