求助OG13的一道邏輯，萬分感謝！！！

encantada

61. 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 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 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 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 flow has been controlled by damming than in rivers that flow freely.
正確答案是選 D
可是我不太明白"explain the contrast"的 contrast 只得是解釋題目中的哪裡啊？ 正確答案給的是說這道題想問"where would explain the contrast between algae levels in slow-moving water resulting from little rain and slow-moving water resulting after a drought?"
可是我還是不明白要考的是甚麼啊？我一開始選的是A

hothlove

就是解释矛盾，你看题目中陈述了两个事情，：1、little rian导致 slow-moving water 然后导致 algae增加 2、在extreme drought的情况下，即使slow moving water 中的algae也减少了。两件事情中是有矛盾的。然后正确答案就是叫你选出来能解释说通这个矛盾的。选项D说extreme drought的情况下，可能会有短时间的断流，所以可以解释1、2之间产生的矛盾。

我也是GMAT小白，才上完XDF的班，不知道说对没，还希望其他大神指教。

encantada

多謝拉！！！