10. Some species of dolphins find their prey by echolocation; they emit clicking sounds and listen for echoes returning from distant objects in the water. Marine biologists have speculated that those same clicking sounds might have a second function: particularly loud clicks might be used by the dolphins to stun their prey at close range through sensory overload.

Which of the following, if discovered to be true, would cast the most serious doubt on the correctness of the speculation described above?

(A) Dolphins that use echolocation to locate distant prey also emit frequent clicks at intermediate distances as they close in on their prey.

(B) The usefulness of echolocation as a means of locating prey depends on the clicking sounds being of a type that the prey is incapable of perceiving, regardless of volume.

(C) If dolphins stun their prey, the effect is bound to be so temporary that stunning from far away, even if possible, would be ineffective.

(D) Echolocation appears to give dolphins that use it information about the richness of a source of food as well as about its direction.（B）

(E) The more distant a dolphin’s prey, the louder the echolocation clicks must be if they are to reveal the prey’s presence to the hunting dolphin.

我觉得Ｂ要想成功的削弱，还需要条件，如果把Ｂ选项改为：The usefulness of echolocation as a means of locating prey depends solely/only on the clicking sounds being of a type that the prey is incapable of perceiving, regardless of volume.

因为如果没有这个限定，B本身排除不了echolocation还有其他辅助功能的可能性。

请那位XDJM给解释一下先。谢谢了。

我又使劲想了想，好像想通了。问题不在于条件的充分性上：

The usefulness of echolocation as a means of locating prey depends on the clicking sounds being of a type that the prey is incapable of perceiving, regardless of volume.

而在于我highlight的部分。该题削弱的是：如果假设成立，那原来的事实必不成立。也就是说，这种echolo擦tion的可用性取决于prey们对他的不可感知性，如果他能把可怜的prey们给搞昏，则其作为locate prey的可行性的前提受到了威胁。

也不知道俺想的对不。

削弱是削弱结论

结论是科学家说echolocation

可以震晕猎物, echolocation-----猎物晕 架了桥梁.

削弱就断开桥梁,说猎物对echolocation没有感知能力, 音量再大也听不见.

echolocation---/--猎物晕

至于echolocation究竟有什么其他辅助作用，没有提级也不用考虑吧.