OG详解-OG9 文法 Q18

Choice C is the best answer. The convention being tested is the use of verb forms within a sentence. The plural verb “study” agrees in number with the plural subject “many.” 
Choice A is incorrect because the singular verb “is studying” doesn’t agree in number with the plural subject “many.” Choice B is incorrect because the singular verb “has studied” doesn’t agree in number with the plural subject “many.” Choice D is incorrect because the singular verb “studies” doesn’t agree in number with the plural subject “many.”