OG12--CR 62

zshaoqing

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.

我选的是E，D项没搞懂，求指教！！

EMMALIULIU

A选项中的“During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.”
OG解释是“Th is indicates one of the consequences of drought, and slightly suggests that this might be due to
a lower algae level. But it does nothing to explain why algae levels might be lower after a drought。”
仔细研读的意思是，在干旱期间，species （以海藻为食物，注意feed on的意思）会下降。A选项只能说明海藻下降，所以吃海藻的生物下降，不能解释为什么极端干旱后海藻数量下降。

-- by 会员 佳佳蔻蔻 (2012/3/6 21:18:13)

弱弱的问一下，以海藻为食物的species下降，我怎么觉得应该导致海藻数量上升啊.......不明白.....

-- by 会员 fifiwong (2012/12/14 23:34:48)

一开始看到A，我脑子里第一反应也是，这个不是在解释algae数量上升么。立刻删了，因为题目要解释为什么下降。

但是你再看看，这里有个“tend to”，只有当食物（这里是algae）先下降，以该食物为生的物种才会“tend to”下降。

满tricky的一个选项，还好这里的误解，不影响排除这个选项。

xxcjc

说得对 如果有人和我一样错选A 那么原因一定是由于理解错了“feed on”的意思 species that feed on algae是说以海藻为食的物种而不是被海藻吃掉的物种。奶奶的 坑死了

fifiwong

A选项中的“During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.”
OG解释是“Th is indicates one of the consequences of drought, and slightly suggests that this might be due to
a lower algae level. But it does nothing to explain why algae levels might be lower after a drought。”
仔细研读的意思是，在干旱期间，species （以海藻为食物，注意feed on的意思）会下降。A选项只能说明海藻下降，所以吃海藻的生物下降，不能解释为什么极端干旱后海藻数量下降。

-- by 会员 佳佳蔻蔻 (2012/3/6 21:18:13)

弱弱的问一下，以海藻为食物的species下降，我怎么觉得应该导致海藻数量上升啊.......不明白.....

youyoumekin

Thanks for your explanation about choice A!

佳佳蔻蔻

D选项的意思是，在extreme drought期间，河水都干枯了，algae完全死了。之后，河流重新有水了，是慢水（in very slow-moving river water），但由于干旱期间死了一大片algae，这下algae数量还是low。

佳佳蔻蔻

A选项中的“During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.”
OG解释是“Th is indicates one of the consequences of drought, and slightly suggests that this might be due to
a lower algae level. But it does nothing to explain why algae levels might be lower after a drought。”
仔细研读的意思是，在干旱期间，species （以海藻为食物，注意feed on的意思）会下降。A选项只能说明海藻下降，所以吃海藻的生物下降，不能解释为什么极端干旱后海藻数量下降。

GWinner

还是觉得D选项中的dry up completely和文章中的in very low moving water相悖，感觉这样的话就没有基于文章的内容来explain了....也不明白为什么A错......> <求赐教啊求赐教!!

ndslyy

问下A的解释是怎么意思啊？什么slightly suggest that this might be due to a lower algae level. 我怎么觉得是会引起algae的上升啊。。。？

FlyUFalcon