Reading Comprehension (RC) | Re: Even more than mountainside slides of mud or snow, naturally occurring

I would be so appreciative if an expert can explain the explanations for this passage. I also have specific questions below.

1. For "The primary purpose of the passage is to explain the" question:
Why exactly is Choice A (qualities that make a particular organism unique) incorrect? Choice A and C seem very similar.

2. For the "It can be inferred from the passage that when aspen groves experience a ???disturbance??? (see highlighted text), such a disturbance" question:
I
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