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.

Q28:

The primary purpose of the passage is to

确认E，题目问的是写做目的。所以E的identify符合题意。

我觉得选D。

这篇文章应该是包含2个方面的：1 生物钟的作用2 生物钟被环境所调节。

E的主要问题是它只提了第2个方面：环境因素对生物钟的影响。

而且limit effectiveness这个词感觉用的不准确。

我也是在D和E之间徘徊了好久，最终选了E，文章开头科学家针对一个现象提出假说，后来试验反驳了这个假说。进而进一步解释文章开头的现象。题干问写作目的，D和E之间区别很小。我最后排除D就觉得D的这个词examine.觉得E的identify这个词用的好。

不过现在觉得还是D好，examine的意思应该是调查，不是测试，下午把他看成测试了，觉得不对。理解成检查D就非常正确了。