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og12- cr62
When there is less rainfall than normal, the water level
of Australian rivers falls and the rivers fl ow more
slowly. Because algae whose habitat is river water
grow best in slow-moving water, the amount of algae
per unit of water generally increases when there has
been little rain. By contrast, however, following a
period of extreme drought, algae levels are low even
in very slow-moving river water.
Which of the following, if true, does most to explain
the contrast described above?
(A) During periods of extreme drought, the
populations of some of the species that feed on
algae tend to fall.
(B) The more slowly water moves, the more
conducive its temperature is to the growth
of algae.
(C) When algae populations reach very high levels,
conditions within the river can become toxic for
some of the other species that normally live
there.
(D) Australian rivers dry up completely for short
intervals in periods of extreme drought.
(E) Except during periods of extreme drought, algae
levels tend to be higher in rivers in which the
fl ow has been controlled by damming than in
rivers that fl ow freely.
答案是D。我选的是A 想不明白啊！ 求指教！ |
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