GRE Reading Comprehension: ETS-GRE阅读ETS - 4JL7SCH868ZL4Z187

It can be inferred from the passage that which of the following has NOT been a factor influencing the extent to which research on mycorrhizal fungi has progressed? A. Lack of a method for identifying mycorrhizal fungi B. Difficulties surrounding laboratory production of specimens for study C. Difficulties ensuing from the high cost and scarcity of superphosphate fertilizers