gwd-1-28

Q25 to Q28:

      Recently biologists have been

            interested in a tide-associated

            periodic behavior displayed by

Line     the diatom Hantzschia virgata, a

  (5)      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

(10)     burrowed in the sand during high

tide, and emerge on the sand sur-

face during the daytime low tide.

Just before the sand is inundated by

the rising tide, the diatoms burrow

(15)     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

(20)     in pressure caused by tidal move-

            ment.  However, when diatoms are

            observed under constant conditions

in a laboratory, they still display

periodic behavior, continuing to bur-

(25)     row on schedule for several weeks.

This indicates that commuter diatoms,

rather than relying on environmental

cues to keep time, possess an inter-

nal pacemaker or biological clock

(30)     that enables them to anticipate peri-

odic changes in the environment.

A commuter diatom has an unusu-

            ally accurate biological clock, a

consequence of the unrelenting

(35)    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

(40)    period of this biological clock is

immutably fixed.  Biologists have

concluded that even though a

diatom does not rely on the envi-

ronment to keep time, environmental

(45)     factors—including changes in the

tide’s hydrostatic pressure, salin-

ity, mechanical agitation, and

temperature—can alter the period

of its biological clock according to

(50)     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[x1] :  the owner cannot

(55)     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

(60)     exposed to the solar-day cycle as

well as to the tidal cycle, and some-

times display both solar-day and

tidal periods in a single behavior.

Commuter diatoms, for example,

(65)     emerge only during those low tides

that occur during the day.

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Q28:

The primary purpose of the passage is to

MY 2 cents, key is D

GWD-1-Q28:

The primary purpose of the passage is to              

1. dispute the influence of environmental factors on the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata —>不光有反对，也有赞成
   



2. describe how certain tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata have changed over time —>看似正确的混淆选项（将选项看完整很重要）
   



3. compare tide-associated behavioral rhythms to solar-day behavioral rhythms in the diatom Hantzschia virgata —>最后一段的细节内容，错
   



4. examine how certain biological and environmental influences affect the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata
   



5. identify certain environmental factors that limit the effectiveness of the biological clock in the diatom Hantzschia virgata —>限制有效性，错
   

我还是觉得这题应该选A，因为原文第一段说有这么一种现象，科学家假设这是因为环境造成的，所以做了一个实验，但实验结果表明不是环境造成了这个现象；第二段说虽然如此，但环境因素也还是有一定作用的。

所以文章整体都是围绕环境因素展开论述的，dispute不是反对，而是争论的意思，

我认为A和C最大的不对是文章讲了两个方面，一个是生物钟的作用，一个是环境因素对生物钟的影响。而A和C只讲了其中的一个方面。primary purpose应该把两个方面都放在上面才对。

清NN指教

同意!