求助！！！想了一天了，还没想通。。。。。。。求高手相助

fannybeibei

Over the last 150 years, large

stretches of salmon habitat have
been eliminated by human activity:
Linemining, 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 disturbancesin
(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

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
   
2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
   
3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
   
4. differentiate the particular effects of various human activities on salmon habitats
   
5. describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction
   
   ED就划线部分那么一小段，把这个作为primary purpose也太夸张了吧！！！。。。。。。。。。。。。。。
   那第二段also那个disturbulance怎么办呀
   

fannybeibei

看了讨论，基本是找字眼挑错的，
有谁能回答我的问题呀

edwardelric

这题明显是e啊，
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.
这句是本文主题句，这个承认吧
再有你说第二段disturbulance，是文章说Large-scale disturbancesin
(25)one locale also have the potential
to alter the genetic structure of
populations in neighboring areas
但是归根在最后还是
归到环境条件稳定时迷路率相对低，但当溪
流受到严重扰乱失迷路率会大大提升。
跟栖息地的丧失还是息息相关，第二段首提出了一个原因，根本的还在第一段啊~

fannybeibei

谢谢
看了分析，现在懂了
是我太弱了。。。英语不好，还有单词不认识，整篇文章看得很晕乎