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I found this passage very difficult.
Any advice on how to attack it will be much appreciated.

Q9 to Q12:
Over the last 150 years, large
stretches of salmon habitat have
been eliminated by human activity:
Line mining, livestock grazing, timber
(5) harvesting, and agriculture as well
as recreational and urban devel-
opment. The numerical effect is
obvious: there are fewer salmon
in degraded regions than in pris-
(10) tine ones; however, habitat loss
also has the potential to reduce
genetic diversity. This is most
evident in cases where it results
in the extinction of entire salmon
(15) populations. Indeed, most
analysts believe that some kind
of environmental degradation
underlies the demise of many
extinct salmon populations.
(20) Although some rivers have
been recolonized, the unique
genes of the original populations
have been lost.
Large-scale disturbances in
(25) one locale also have the potential
to alter the genetic structure of
populations in neighboring areas,
even if those areas have pristine
habitats. Why? Although the
(30) homing instinct of salmon to their
natal stream is strong, a fraction
of the fish returning from the sea
(rarely more than 15 percent)
stray and spawn in nearby
(35) streams. Low levels of straying
are crucial, since the process
provides a source of novel
genes and a mechanism
by which a location can be
(40) repopulated should the fish
there disappear. Yet high rates
of straying can be problematic
because misdirected fish may
interbreed with the existing stock
(45) to such a degree that any local
adaptations that are present
become diluted. Straying
rates remain relatively low when
environmental conditions are
(50) stable, but can increase dramati-
cally when streams suffer severe
disturbance. The 1980 volcanic
eruption of Mount Saint Helens,
for example, sent mud and debris
(55) into several tributaries of the
Columbia River. For the next
couple of years, steelhead trout
(a species included among the
salmonids) returning from the
(60) sea to spawn were forced to
find alternative streams. As
a consequence, their rates of
straying, initially 16 percent,
rose to more than 40 percent
(65) overall.
Although no one has quantified
changes in the rate of straying
as a result of the disturbances
caused by humans, there is no
(70) reason to suspect that the effect
would be qualitatively different
than what was seen in the
aftermath of the Mount Saint
Helens eruption. Such a dra-
(75) matic increase in straying from
damaged areas to more pristine
streams results in substantial
gene flow, which can in turn lower
the overall fitness of subsequent
generations.
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Q9:
The primary purpose of the passage is to
A. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
B. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
C. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
D. differentiate the particular effects of various human activities on salmon habitats
E. describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction
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Q10:
It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which
A. pristine streams that are near polluted streams become polluted themselves
B. the particular adaptations of a polluted stream's salmon population can be preserved without dilution
C. the number of salmon in pristine habitats decreases relative to the number in polluted streams
D. an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
E. the extinction of the salmon populations that spawn in polluted streams is accelerated
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Q11:
According to the passage, human activity has had which of the following effects on salmon populations?
A. An increase in the size of salmon populations in some previously polluted rivers
B. A decline in the number of salmon in some rivers
C. A decrease in the number straying salmon in some rivers
D. A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams
E. A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction
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Q12:
The author mentions the "aftermath of the Mount Saint Helens eruption" (lines 73-74) most likely in order to
A. provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct
B. indicate the extent to which the disturbance of salmon habitat by human activity in one stream might affect the genetic structure of salmon populations elsewhere
C. provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured
D. show how salmons' homing instinct can be impaired as a result of severe environmental degradation of their natal streams
E. show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance
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