请教GWD-TN-24中的几题阅读

marco_polo

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban development.  The numerical effect is obvious:  there are fewer salmon in degraded regions than in pristine 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 populations.  Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. Although some rivers have been recolonized, the unique genes of the original populations have been lost.
      Large-scale disturbances in 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 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 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 repopulated should the fish there disappear.  Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.  Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance.  The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris into several tributaries of the Columbia River.  For the next couple of years, steelhead trout (a species included among the salmonids) returning from the 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 overall.
      Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, 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.  Such a dramatic 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.

GWD1-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
答案是D，我选E，请教一下我错在哪里

 Recently biologists have been interested in a tide-associated periodic behavior displayed by the diatom Hantzschia virgata, a microscopic golden-brown alga that inhabits that portion of a shoreline washed by tides (the intertidal zone). Diatoms of this species, sometimes called “commuter” diatoms, remain burrowed in the sand during high tide, and emerge on the sand surface during the daytime low tide. Just before the sand is inundated by the rising tide, the diatoms burrow again.  Some scientists hypothesize that commuter diatoms know that it is low tide because they sense an environmental change, such as an alteration in temperature or a change in pressure caused by tidal movement.  However, when diatoms are observed under constant conditions in a laboratory, they still display periodic behavior, continuing to burrow on schedule for several weeks. This indicates that commuter diatoms, rather than relying on environmental cues to keep time, possess an internal pacemaker or biological clock that enables them to anticipate periodic changes in the environment. A commuter diatom has an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away.
      This is not to suggest that the period of this biological clock is immutably fixed.  Biologists have concluded that even though a diatom does not rely on the environment to keep time, environmental factors—including changes in the tide’s hydrostatic pressure, salinity, mechanical agitation, and temperature—can alter the period of its biological clock according to changes in the tidal cycle.  In short, the relation between an organism’s biological clock and its environment is similar to that between a wristwatch and its owner:  the owner cannot make the watch run faster or slower, but can reset the hands.  However, this relation is complicated in intertidal dwellers such as commuter diatoms by the fact that these organisms are exposed to the solar-day cycle as well as to the tidal cycle, and sometimes display both solar-day and tidal periods in a single behavior. Commuter diatoms, for example, emerge only during those low tides that occur during the day.

GWD1-Q25:
The passage suggests which of the following about the accuracy of the commuter diatom’s biological clock?

A. The accuracy of the commuter diatom’s biological clock varies according to changes in the tidal cycle.
B. The unusual accuracy that characterizes the commuter diatom’s biological clock is rare among intertidal species.
C. The commuter diatom’s biological clock is likely to be more accurate than the biological clock of a species that is subject to less intense environmental pressures.
D. The commuter diatom’s biological clock tends to be more accurate than the biological clocks of most other species because of the consistency of the tidal cycle.
E. The accuracy of the commuter diatom’s biological clock tends to fluctuate when the diatom is observed under variable laboratory conditions.
答案C，可是文章里并没有明显提到有commuter diatom’s biological clock何其他物种的biological clock的比较啊。难道是“A commuter diatom has an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away.”