[求助]一道OG上Diagnostic test的题目，没有讨论过

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 rise 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 weaken the argument?

B   If the Arctic warming continues, guillemots' current predators are likely to succeed in extending their own range farther north.

D   If the Arctic warming continues, much of the thin ice in the southern Arctic will disppear.

这道题OG 上给出的答案是D。OG的解释也很明确，是因为并非扩大领域，而是替代了南部的领域。

可是我不理解的是B为什么不对呢？B成立的话说明捕食G这种鸟类的动物也向北扩展，那么G为了躲避捕食者，不就不能扩大向北的领域了吗？

请NN们指教！

是我的问题很傻吗？没人关心呢！

再问再问！跪求解释！