OG详解-OG3 文法 Q10

Choice C is the best answer. The sentence emphasizes the decline in unique apple varieties in the US and specifies why this decline occurred, noting that thousands of apple varieties were lost because US farmers started mainly growing the same few unique varieties. 
Choice A is incorrect. The sentence introduces the Lost Apple Project; it doesn’t emphasize the decline in unique apple varieties in the US and specify why this decline occurred. Choice B is incorrect. While the sentence emphasizes the decline in unique apple varieties in the US, it doesn’t explain why this decline occurred. Choice D is incorrect. The sentence emphasizes the general decline of crop varieties in the mid-1900s; it doesn’t emphasize the specific decline in unique apple varieties in the US.