36套EX9

jeffrey-jinUID: 2711580

Mycorrhizal fungi infect more plants than do any
other fungi and are necessary for many plants to thrive,
but they have escaped widespread investigation until   
Line  recently for two reasons. First, the symbiotic association
5  is so well-balanced that the roots of host plants show no
damage even when densely infected. Second, the fungi
cannot as yet be cultivated in the absence of a living
root. Despite these difficulties, there has been important
new work that suggests that this symbiotic association
10  can be harnessed to achieve more economical use of
costly superphosphate fertilizer and to permit better
exploitation of cheaper, less soluble rock phosphate.
Mycorrhizal benefits are not limited to improved
phosphate uptake in host plants. In legumes,   
15  Mycorrhizal inoculation has increased nitrogen fixation
beyond levels achieved by adding phosphate fertilizer
alone. Certain symbiotic associations also increase the
host plant‘s resistance to harmful root fungi.   

9. 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

答案AC，B为啥是因素之一啊？文章没说

413麦兜兜UID: 3120121

Second, the fungi  cannot ....cultivated in absence of living root~
应该是说living root 在实验室是不能被模仿的~所以形成了一种限制的条件