OG详解-OG6 文法 Q23

Choice B is the best answer. The convention being tested is subject-verb agreement. The plural verb “increase” agrees in number with the plural subject “toxins.”
Choice A is incorrect because the singular verb “is increasing” doesn’t agree in number with the plural subject “toxins.” Choice C is incorrect because the singular verb “increases” doesn’t agree in number with the plural subject “toxins.” Choice D is incorrect because the singular verb “has increased” doesn’t agree in number with the plural subject “toxins.”