Some species of dolphins find their prey by echo-

location; 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.

(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

答案是B。但感觉C更好。因为问的是”cast the most serious doubt on the correctness of the speculation described above?“。而题中的第一点已经是事实，而第二点才是speculation ：”。。。stun their prey at close range through sensory overload.“。

还是B好，B彻底否定了stun得必要性