OG详解-OG9 文法 Q16

Choice C is the best answer. The convention being tested is the use of possessive determiners. The plural possessive determiner “their” agrees in number with the plural conjoined noun phrase “Watson and Crick” and thus indicates that the findings were those of Watson and Crick. 
Choice A is incorrect because “they’re” is the contraction for “they are,” not a possessive determiner. Choice B is incorrect because “it’s” is the contraction for “it is” or “it has,” not a possessive determiner. Choice D is incorrect because the singular possessive determiner “its” doesn’t agree in number with the plural conjoined noun phrase “Watson and Crick.”