复习中的逻辑疑惑，请各位大牛帮忙

harvard2012

1. during the month of June, crabs arrive on Atlan's beaches to lay eggs. Certain waders depend onn these eggs for food during their annual spring migration to their northern breeding grounds. Atlan's crab population has declined recently. this decline, nevertheless, poses no serious threat to the migrating wader populations: by remaining longer in Atlan, the birds will be able to eat enough crab eggs to allow them to complete their migration. 问weaken结论？
(e) the earlier in the seaso the waders complete their migration, the more likely they are to breed successfully.
正确答案就是E，我是用排除法选正确的，但是不知道E的逻辑是怎样的啊

2. black americans are, on the whole , about twice as likely as white americans to develop high blood pressure. this likelihood also holds for westernized black africans when compared to white africans. researchers have hypothesized that this predisposition in westernized blacks may reflect an interaction between western high-salt dites and genes that adapted to an environmental scarcity of salt.
which of the following statements about present-day, westernized black africans, if true, would most ten to conirm the researchers' hypothesis?
(a) the blood pressures of those descended from peoples situated throughout their history in Senegal and Gambia, hwere salt was always available, are low.
(d) blood pressures are low among the Yoruba, who, throughout their history, have been sitated far inland from sources of sea salt and far south of Saharan salt mines.
正确是A，我选D， 完全没懂啊

3.sodus university's fund-raisers succeeded in getting donations from 85 percent of the potential donors they contacted. this success rate, exceptionallyh high for university fund-rasiers, does not indicate that they were doing a good job. on the contrary, because the people most likely to donate are those who have donated in the past, good fund-raisers continually try less-likely prospected in an effort to expand the donor base. the high success rate shows insufficient cancassing effort.
问support？
（a) sodus unviersity's fund-raisers were successful in their contacts with potential donors who had never given before aobut as frequently as were fund-raisers for other universities in their contacts with such people.
完全没看懂这个正确选想啊

harvard2012

哈哈是啊，应该换个名了

HarrisZheng

看你这id ，2012应该是赶不上了

1 结论是鸟还是能吃饱，前提是能呆足够长时间
E 说鸟到得越早，越有利于繁衍 也就是说鸟不能呆足够长的时间 前提被weaken了，结论也就被weaken了

2美国黑人比白人血压高，西化黑人比非洲白人血压高，因此，推断西化黑人血压高是吃盐多和基因不耐盐的结果
即 吃盐多（A）+不耐盐基因（B）--> 血压高（C）
A选项 是weaken结论的  因为 A+B-->血压低（非C）
D选项 盐少（非A）+ B-->血压低（非C） 无A因则无C果 支持结论

3 数学题 http://forum.chasedream.com/GMAT_CR/thread-641512-1-1.html