阅读下列短文，从所给的四个选项中，选出最佳答案。Aquestionoftenputtospecialistsonfishesis"Howlongdofisheslive?"Specialistsfinditdifficulttogiveanexactanswerthoughits

　　阅读下列短文，从所给的四个选项中，选出最佳答案。

　　Aquestionoftenputtospecialistsonfishesis"Howlongdofisheslive?"Specialistsfinditdifficulttogiveanexactanswerthoughitseemstobeaverysimplequestion.Actuallythisquestionisnotsosimpleasitseems.Therearethousandsofdifferentkindsoffishesandtheyaredifferentfromoneanotherinsizeandlife-span(寿命).Moreover(此外)itisnoteasytofindoutjusthowlongafishlivesinitsnaturalstate.

　　Wecanfindouthowoldafishisbystudyingitssizebutwecannotsayhowmuchlongeritwouldhavelivedifwehadnotcaughtit.

　　Wemayfeedfishesandrecordtheirlife-spanbutwecannotbesurethatthisisthelengthoftimetheywouldhavelivediftheyhadbeenleftalone.

　　Wemaymakemarkingstoshowhowfastthefishesgrowsothatwecancalculate(计算)theageofthelargestonrecordbutunlessthislargefishdiesofoldagewearestillnotinapositiontoknowitsnaturallife-span.

　　Unlikehumanbeingsfishesdonotstopgrowingwhentheyreachmaturity(成年).Theycontinuetogrowaslongastheylivealthoughtherateofgrowthslowsdowningrown-upfishes.

　　(1)Whatspecialistsfeeldifficulttoknowis________.

　　[]

　　A.theageoffishes

　　B.thesizeoffishes

　　C.thelife-spanofthefisheshefeeds

　　D.thelife-spanoffisheslivingfreely

　　(2)Wemayknowafish'sagebystudying__________.

　　[]

　　A.howheavyitis

　　B.howbigitis

　　C.itsrateofgrowth

　　D.thepositionitlives

　　(3)Whichofthefollowingistrue?

　　[]

　　A.Theheavierafishistheolderitis.

　　B.Ifweletafishalonewecanhardlyknowitslife-span.

　　C.Oldasafishisitwillkeepgrowingfastbeforeitsdeath.

　　D.Whenamanisoldhewillgrowmuchmoreslowlythananoldfish.

　　(4)Whichofthefollowingisnottrue?

　　[]

　　A.Ifwefeedafishfromitsbirthtoitsdeathwecorrectlyrecorditslife-span.

　　B.Afish'sweighthasnothingtodowithitslife-span.

　　C.Humanbeingsstopgrowingwhentheyhavegrownup.

　　D.Specialistscanknowhowfastafishgrowsbuttheydonotknowhowlongitwilllive.

　　(5)Whichofthefollowingdo