求助两道逻题目~~~

清北负娇人

During the month of May, crabs arrive on Delaware's beaches to lay eggs. Certain shorebirds depend on these eggs for food during their annual spring migration to their northern breeding grounds. Delaware's crab population has declined recently. This decline, however, poses no serious threat to the migrating shorebird populations: by remaining longer in Delaware, the birds will be able to eat enough crab eggs to allow them to complete their migration.

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

• A No other food available to the birds on Delaware's beaches is as nutritious and accessible as are crab eggs.
• B The decline in the number of crabs on Delaware's beaches is due to the harvesting of crabs by humans.
• C There are more crabs on Delaware's beaches than in any other area that the migrating birds pass through.
• D The crabs do not conceal their eggs.
• E The earlier in the season the shorebirds complete their migration, the more likely they are to breed successfully.
  

Guillemots are birds of Arctic Regions. They feed on fish that gather beneath thin sheets of floating ice, and they nest on nearby land. Guillemots need 80 consecutive snow-free days in a year to raise their chicks, so until average temperatures in the Arctic began to rice recently, the guillemots' range was limited to the southernmost Arctic coast. Therefore, if the warming continues, the guillemots' range will probably be enlarged by being extended northward along the coast.

Which of the following, if true, most seriously weakens the argument?
A. Even when the warming trend continues, there will still be years in which guillemot chicks are killed by an unusually early snow.
B. If the Arctic warming continues, guillemots' current predators are likely to succeed in extending their own range farther north.
C. Guillemots nest in coastal areas, where temperatures are generally high than inland areas.
D. If the Arctic warming continues, much of the thin ice in the southern Arctic will disappear.
E. The fist that guillemots eat are currently preyed on by a wilder variety of predators in the southernmost Arctic regions than they are farther north.


第二题的一个premise是"by being extended northward along the coast", argument是“guillemots' range will probably be enlarged”，g鸟向南方扩张的时候，g鸟的栖息地却没有扩张则可以削弱，答案为D没有问题。
同理，我觉得第一题的结论是 “the birds will be able to eat enough crab eggs to allow them to complete their migration”， 前提是“by remaining longer in Delaware“，如果蟹没有把蛋藏起来，说明蛋的数量供不应求的等再久也没用，所以我推断出答案选D。            
可为什么结论是鸟呆的时间更久而不是鸟将找到足够多的蛋呢？？？

zsesz

most seriously weaken类要选择能够割裂因果关系的选项：原题干需要削弱的命题是：crabs（egg）数量减少不会影响鸟的数量，因为鸟可以通过延长觅食的时间来获取充足的食物。
因此需要割裂的因果关系是：延长觅食时间——不影响鸟的数量。选E