| 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, ....
GWD1-Q28:
The primary purpose of the passage is to
A dispute the influence of environmental factors on the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata
B describe how certain tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata have changed over time
C compare tide-associated behavioral rhythms to solar-day behavioral rhythms in the diatom Hantzschia virgata
D examine how certain biological and environmental influences affect the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata
E identify certain environmental factors that limit the effectiveness of the biological clock in the diatom Hantzschia virgata
看过讨论, 觉得D虽然讲了文章的意思,但是问题问的是the primary purpose,不是main idea?我摘录下来的内容都是作者阐述的其他科学家的观点,从however后面开始批判.所以,我觉得主要的意图就应该选A啊? | 
