GWD1－27

  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 :  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.

我也选了C，over the long term 长期来看。

但是答案是B，line 25 for several weeks.

可能是这个原因吧。

GWD 1-27 的答案我认为应该是C,　Ｂ和Ｃ不只是在长期短期上不同，Ｂ的意思是在短期内 periodic behavior 一致，既然是constant laboratory conditions ， periodic behavior 当然一致，作者想比较的是在constant laboratory conditions 和natural conditions情况下形为有没有不同，而并非在constant laboratory conditions 下 periodic behavior 一不一致．

我觉得这题肯定是选B的。

解释：如果several weeks是长期，长期包含了短期，B正确

     如果several weeks是短期，B也正确

而C是不一定正确。太过于绝对。