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.
D Correct. Th is statement properly identifi es something that helps explain the contrast. According
to the information given, the habitat of the algae under discussion is river water. If the river dries
up, the algae will probably not survive. Th en after the drought, algae population levels would
likely take a while to rise again.
我就不明白TM万一这水草是两栖植物呢，GMAC不能这样考常识吧。认为我太绞真的或者认为这个问题实在太无聊的童鞋可以抽我，右边脸啊，左边长了个痘痘。
题目已经告诉你algae的habitat是river water了 已经限定死了
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