Ex 9 一句话的理解 求指教

lionelfengUID: 2371460

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
   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

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

我觉得蓝色部分是对应重复的，求指教！

dandelionttUID: 2852417

我怎么不觉得是重复的呢

lionelfengUID: 2371460

2# dandeliontt
我是这样想的：
文章说 作为研究困难之一，第一，在寄主根部找不到任何的破坏，即使它已经被感染的很深了，也就是说我们研究起来无法知道哪个被感染，哪个没有被感染，换言之就是无法identify这种真菌，望指教

lionelfengUID: 2371460

求指教