[求助]OG10—17—102

Passage 17

Australian researchers have discovered electroreceptors

(sensory organs designed to respond to electrical fields)

clustered at the tip of the spiny anteater’s snout. The

researchers made this discovery by exposing small areas of

(5) the snout to extremely weak electrical fields and recording

the transmission of resulting nervous activity to the brain.

While it is true that tactile receptors, another kind of

sensory organ on the anteater’s snout, can also respond to

electrical stimuli, such receptors do so only in response to

( 10) electrical field strengths about 1,000 times
            greater than

those known to excite electroreceptors.

Having discovered the electroreceptors, researchers are

now investigating how anteaters utilize such a sophisticated

sensory system. In one behavioral experiment, researchers

(15) successfully trained an anteater to distinguish between

two troughs of water, one with a weak electrical field

and the other with none. Such evidence is
            consistent with

researchers’ hypothesis that anteaters use electroreceptors

to detect electrical signals given off by prey; however,

( 20) researchers as yet have been unable to detect electrical

signals emanating from termite mounds, where the favorite

food of anteaters live. Still, researchers have observed

anteaters breaking into a nest of ants at an oblique angle

and quickly locating nesting chambers. This ability quickly

(25)to locate unseen prey suggests, according to the researchers,

that the anteaters were using their electroreceptorsto locate

the nesting chambers.

(A) Researchers had difficulty verifying the existence of electroreceptors in the anteater because electroreceptors respond to such a narrow range of electrical field strengths.

(B) Researchers found that the level of nervous activity in the anteater’s brain increased dramatically as the

strength of the electrical stimulus was increased.

(C) Researchers found that some areas of the anteater’s snout were not sensitive to a weak electrical stimulus.

(D) Researchers found that the anteater’s tactile receptors were more easily excited by a strong

electrical stimulus than were the electro receptors..

(E) Researchers tested small areas of the anteater’s snout in order to ensure that only electroreceptors were

responding to the stimulus.

不明白C是怎么推出来的？