You're not alone in asking about question #6...
Question 6
Question 6 asks about the function of the following sentence:
"Anecdotal reports of population decreases cannot distinguish among these possibilities, and some amphibian populations have shown strong fluctuations in the past."
(B) states that the purpose of this sentence is:
The
previous sentence is:
"Even here, distinctions must again be made among declines that are only apparent (in the sense that they are part of habitual cycles or of normal fluctuations), declines that take a population to some lower but still acceptable level, and those that threaten extinction (e.g., by taking the number of individuals below the minimum viable population)."
The
assertion made in this sentence is that "distinctions must again be made" among different kinds of declining species. The sentence discussed in question 6 says that we
cannot make these distinctions using anecdotal reports.
So, the sentence in question doesn't
cast doubt on the assertion made in the previous sentence -- the author still believes that we
should make these distinctions. He/she just says that we
can't do that with certain kinds of reports.
Compare that to (C):
In the first paragraph, the environmentalists claim that reports of amphibian population decline indicate "general and catastrophic environmental degradation due to human activity." By stating that certain reports can't give us all of the information we need to evaluate that claim, the author is raising an objection to the environmentalists' view.
To answer @coinsy's question -- yes, the first paragraph specifies that the environmentalists will only hold their claim if the reported population declines are real. However, being "real" doesn't help to determine which
kind of population decline is occurring -- is it part of a normal fluctuation? Or one that will take the population to a low but acceptable level? Or will it truly threaten extinction?
In saying that this determination cannot be made, the author raises an objection even if amphibian populations are declining, as reported.
(C) is the correct answer to question 6.
I hope that helps!