参考答案

正确答案：C
Thisroomisdevotedtoelectricfish.[29]Thefishinthetankbehindmecanproduceastrongjoltofelectricitytoshockitsprey,butmostofthefishinhereproduceonlyweakelectricalimpulsesthatareusefulfornavigating,locatingfood,andevenforcommunicating.Theknifefishisagoodexample.Thisfishnavigatesusingtinyreceptorsintheskinthataresensitivetoelectricalimpulses.[30]Theknifefishproducesanelectricalsignal,andthereceptorsinitsskinletitknowwhenthesignalisdistortedbyatreeroot,orsomeotherobstacle,soitcangoaroundit.Fishalsousetheabilitytoproduceanddetectelectricalimpulsestocommunicate.Theycantelleachotherwhatspeciestheybelongto,howbigtheyare,andwhetherthey'remaleorfemale.[31]Wehaveatankherethat'sspeciallyequippedtoconverttheinaudiblesignalsthefishproduceintosoundsyoucanhearwhenyouputontheseheadphones.IurgeyoualltolisteninwhenI'mdonespeaking.Nowhavealookattheelectricrays.Raysareespeciallyinterestingtomedicalresearchersbecauseoftheorganstheyusetoproduceelectricity.Theseorganscontainachemicalthatcarriessignalsfromonenerveendingtothenext,notonlyinrays,butalsoinpeople.Bystudyingtheseorgans,scientistshopetolearnmoreaboutdiseasesthatinterruptthetransmissionofimpulsesfromonenervetoanother.29.Whatisthetalkmainlyabout?30.Whatdoestheknifefishuseelectricityfor?31.Whydoesthespeakersuggestputtingonheadphones?【解题思路】四项均提到了鱼，而且很可能与发电鱼有关。【解析】短文开头说这是个用于研究发电鱼的房间，然后介绍说后面鱼缸里的鱼可以产生强电流使猎物休克，但大多数的鱼只能发出对导航、确定食物甚至交流有用的弱电流。短文后面分别介绍了这几种应用，因此本篇短文主要谈的是一些鱼怎样运用电流，故选C。