OG--12--62

onedayling

When there is less rainfall than normal ,the water level of Australian rivers falls and the rivers flow more slowly.Because algae whose habitat is river water grow best in slow-moving water,the amount of algae per unit of water generally increase 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 period 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 conductive 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 period of extreme drought.

(E) Except during periods of extreme drought,algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.

这题答案是D~  对OG的分析，A项的解释是 some species 的fall会导致 algae数量的减少~

明明是会导致增多的嘛~怎么OG说是减少啊？

taoyeye

可不可以理解成由于algae数量的减少，导致some species 减少了?因为没食物了

sisucsxs

这里 the species that feed on algae 意思是，algae 是以the species 为食物的，没

食物了，所以导致algae 减少

sisucsxs

不过我也有疑问，为什么A不对。
OG 的解释是 it 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.

请楼下解答~

tongxiaolong

没看过og的解释, 不知道这个题目有什么问题?
A不可能是答案啊,
题目中描述的矛盾在于:
algae在slow moving river环境中数量多, 但是在大旱之后, 即使在slow moving river中,

algae数量也少
A说, 大旱之后, 以algae为食的物种数量下降, 那么回到slow moving river的环境, algae的

数量应该增加, 而不回像题目说的那样, 数量减少, 所以没有解决矛盾, 因此不是答案

netmovr

以我浅见，不选A的原因：

1、A中出现some，many类似的词汇一般都不会是选项。因为一般不够意思彻底。

2、本题中A：说以Algea为食的某些物种由于干旱而减少，可能是由于Algea数量减少引起的，但是这不是解释原因呀。原因是要你解释：为什么在干旱前后，Algea的数量会发生巨变。

再看D。注意题目中这句话：the amount of algae per unit of water generally increase when there has been little rain。就是说少雨、多雨，Algea数量都会增加，只是快慢的事。

但是干旱后，数量却减少了。如D所说，极度干旱引起的河流干涸，这才是导致Algea巨变的原因哦。

PS： feed on 是“以...为食物”。

小萧笑

 A的解释是说：在一段时间的极度干旱之后，以algae为食的一些物种数量下降。（按照OG的说法，这个为干旱后的一种结果）
那从这个结果可以猜想：可能是因为极度干旱，algae减少，才导致这些以alage为食的物种没有食物来源，最终表现为数量下降。但是这个猜想只能推出algae数量减少，却没有办法解释algae为什么在干旱后减少。