求教 TT GWD-14-12

lguo1

    

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.

    

Q12:

    

The author mentions the “aftermath of the Mount
Saint Helens eruption” (lines 73-74) most likely in order to

    

 

    

1. provide
        an example of the process that allows the repopulation of rivers whose
        indigenous salmon population has become extinct
2. 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
3. provide
        a standard of comparison against which the impact of human activity on the
        gene flow among salmon populations should be measured
4. show
        how salmons’ homing instinct can be impaired as a result of severe
        environmental degradation of their natal streams
5. show
        why straying rates in salmon populations remain generally low except when
        spawning streams suffer severe environmental disturbance

The given correct answer is C.   But I feel B is  good  too.   I guess the key here is whether "indicate the extent to which" in B or "provide a standard of comparison against which" in C  is more appropriate.   But how can I judge from the original  article? 

The original article states that  "there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount
        Saint
    Helens eruption".  看起来 B 和 C 都说得过去啊?

Thanks!  

nickynicky

我觉得答案应该是B，C完全错了。

怎么这么多阅读的大案都不对？我要死掉了，马上要考试，到哪里去confirm这些答案？