OG详解-OG2 文法 Q5

Choice C is the best answer. The convention being tested is the use of plural and possessive nouns. The singular possessive noun “screw’s” and the plural noun “threads” correctly indicate that there is only one screw and it has multiple threads. 
Choice A is incorrect because the context requires the plural noun “threads,” not the singular possessive noun “thread’s.” Choice B is incorrect because the context requires the singular possessive noun “screw’s,” not the plural possessive noun “screws’.” Choice D is incorrect because the context requires the singular possessive noun “screw’s” and the plural noun “threads,” not the plural noun “screws” or the plural possessive noun “threads.’”