B

Itisnaturalthatchildrenarecuriousabouttheworldaroundthem.Forexample,theywanttoknowhowtheirheartsbeat.Theywanttoknowwhytheoceanwatertastessalty.

Aschildrengrowup,theybecomecuriousaboutdifferentkindsofthings.Whentheyarebabies,theyareinterestedinthepartsoftheirbodiesandinthesmilesoftheirmothers.Theybecomeinterestedinthephysicalworldaroundthem:theplants,theanimals,thesky.Later,theybecomeinterestedinthethingsthatpeoplehavemade:wheels,bicycles,cars.Andwhentheyareadults,theircuriositycontinues.Sometimesthiscuriosityleadstoacareer(生涯、职业)inscience.

Scientistsspendtheirlivestryingtofindoutabouttheworld．Thosewhoworkwiththeearthsciencesstudytheearth,theoceans,andtheskies.Otherscientistswhostudylivingthingsworkwiththebiologicalsciences.Athirdgroupofscientistsstudythephysicalsciences,e.g.physics,chemistry.

Thesescientistshavealreadydiscoveredalotaboutourworld．Forexample,theytelluswhyyourheartbeatsfastwhenyourun.Theysaythatwhenyouarequiet,yourheartnormallybeatssixty-fiveorseventy-fivetimesaminute.Yourheartisapump(泵)thatpumpsbloodtoallpartsofthebody.Thebloodcarriesoxygenandnutrition.Whenyourun,yourmusclesworkveryhardandusethenutritionthatthebloodcarriestothem.Themusclesneedoxygen,too.Soyourbrainsendsasignaltotheheart.Thesignalmeansthatthemusclesneedmorenutritionandoxygen.Thentheheartbeatsfastandsendsbloodquicklytothemuscles.Itmaybeat90to140timesaminute.

Ofcourse,scientistscannotanswerallofourquestions.Ifweask,“Whydoestheoceanwatertastesalty”scientistswillsaythatthesaltcomesfromrocks.Whenarockgetsveryhotorverycold,itcracks.Rainfallsintocracks.Therainthencarriesthesaltintotheearthandintotherivers.Theriverscarrythesaltintotheocean.Butthenweask,“Whathappenstothesaltintheocean?Theoceandoesnotgetsaltiereveryyear.”Scientistsarenotsureabouttheanswertothisquestion.

Weknowalotaboutourworld,buttherearestillmanyanswersthatwedonothave,andwearecurious.

45．Accordingtothepassage,whichofthefollowingstatementsistrue?

　 A．Peop