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.
27. According to the passage, the periodic behavior displayed by commuter diatoms under constant laboratory conditions is characterized by which of the following?
答案选B，我选C。原文中，21－25行并没有提到长期短期的问题，C虽然也不确切，但B明显暗示了长期的观察结果将inconsistet.所以我觉得应该选C。想问问大家的想法。
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。
JerryGuan 发表于 2004-10-7 23:11
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。 ...
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