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！！

Rainbeauty

D想我觉得就跟这个题不是很相干。。。

wangcaroline

楼上的， D 是答案吧， 没有水了，海藻还怎么活啊。
E是不相关的， 讨论的是Except during periods of extreme drought， 而题目是问during periods of extreme drought

zshaoqing

sorry I cannot type Chinese here
thank you for the explaination about E, I got it
but the main body says " even in very-slow water". it has water there. why said it's completely drought?

Zoil

very-slow water和completely drought都是extreme drought的表现。很有可能完全干了以后，海藻全死了，即使后来有一点点水，活过来的也不多了。

zshaoqing

谢谢哈~！！
脑袋要转不过来了。。。

FlyUFalcon

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。。。？

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!!

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。