GWD14的一道逻辑题！！求解释。。

11. GWD17-Q11:

Some species of Arctic birds are threatened by recent sharp increases in the population of snow geese, which breed in the Arctic and are displacing birds of less vigorous species.Although snow geese are a popular quarry for hunters in the southern regions where they winter, the hunting season ends if and when hunting has reduced the population by five percent, according to official estimates.Clearly, dropping this restriction would allow the other species to recover.

Which of the following, if true, most seriously undermines the argument?

A.Hunting limits for snow geese were imposed many years ago in response to a sharp decline in the population of snow geese.

B.It has been many years since the restriction led to the hunting season for snow geese being closed earlier than the scheduled date.

C.The number of snow geese taken by hunters each year has grown every year for several years.

D.As their population has increased, snow geese have recolonized wintering grounds that they had not used for several seasons.

E.In the snow goose’s winter habitats, the goose faces no significant natural predation.

答案是B。
题目读不懂（下划线部分），麻烦大家解释一下吧！！
有一点我很奇怪，题中突然蹦出来个restriction。。。之前没说有什么限制啊？？

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calvinmonkey发表于 2013-5-20 06:34 | 只看该作者

首先是题目的意思：最近急剧上升的SG的数量威胁到了一些北极鸟物种的生存。SG在北极觅食，并且正在取代其它较弱的鸟类物种。在猎人冬天居住的南部地区，虽然SG是很受欢迎的猎物，但据官方人士估计，如果狩猎活动致使SG数量减少5%，狩猎季便结束了。因此，取消这项限制将使得其它种类的鸟得以恢复。划线部分：where they winter中的they修饰的是hunters而不是SG birds（指代原则：不能隔成分指代），winter活用做动词，表“过冬”，即在冬天，hunters会到比较温暖的南部地区过冬。If and when是限制条件。对if的理解是：如果（if）狩猎活动导致鸟的数量减少5%，狩猎季便结束了。对when的理解是：官员估计，一般情况下当鸟的数量减少5%时，狩猎季也差不多结束了。后面出现
official，指出5%这一上限（restriction）是官方的规定。所以原文总结：如果政府允许hunters在hunting season狩猎更多的SG，便能有效的保护其它鸟种。
B选项：这一限制曾经致使狩猎季早于规定日期前结束，但最近多年狩猎季并没有因为该限制而提前结束。道理：之前该限制导致狩猎季提前结束，是因为If的限制，即狩猎季还没有结束，SG的数量就已经下降了5%了，因此根据官方规定，此时狩猎季不得不提早结束。但最近几年，一直到官方规定狩猎季结束的日期，SG的数量才刚下降5%（也有可能还没下降5%这么多）。猎人在整个狩猎季之间都在打猎，说明猎人狩猎的能力已经发挥到最大了，即使官方将5%的限制上升至10%，在一定的狩猎期季节间，猎人也不能再猎取更多的SG了，因此不能起到保护其它鸟种的作用，从而weaken the argument。
希望能对你有帮助。

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