题目内容：
根据以下材料，回答题
WhyDon'tBabiesTalkLikeAdults?
Overthepasthalf-century,scientistshavesettledontworeasonabletheoriesrelatedtobabytalk.Onestatesthatayoungchild'sbrainneedstimetomasterlanguage,inthesamewaythatitdoestomasterotherabilitiessuchasphysicalmovement.Thesecondtheorystatesthatachild'svocabularylevelisthekeyfactor.Accordingtothistheory,somekeystepshavetooccurinalogicalsequencebeforesentenceformationoccurs.Children'smathematicalknowledgedevelopsinthesameway.
In2007,researchersatHarvardUniversity,whowerestudyingthetwotheories,foundacleverwaytotestthem.Morethan20,000internationallyadoptedchildrenentertheU.S.eachyear.Manyofthemnolongerheartheirbirthlanguageaftertheyarrive,andtheymustlearnEnglishmoreorlessthesamewayinfantsdo--thatis,bylisteningandbytrialanderror.Internationaladopteesdon'ttakeclassesoruseadictionarywhentheyarelearningtheirnewtongueandmostofthemdon'thaveawell-developedfirstlanguage.Allofthesefactorsmakethemanidealpopulationinwhichtotestthesecompetinghypothesesabouthowlanguageislearned.
NeuroscientistsJesseSnedeker,JoyGerenandCarissaShaftostudiedthelanguagedevelopmentof27childrenadoptedfromChinabetweentheagesoftwoandfiveyears.ThesechildrenbeganlearningEnglishatanolderagethanUSnativesandhadmorematurebrainswithwhichtotacklethetask.Evenso,justaswithAmerican-borninfants,theirfirstEnglishsentencesconsistedofsinglewordsandwerelargelybereft(缺乏的)offunctionwords,wordendingsandverbs.TheadopteesthenwentthroughthesamestagesastypicalAmerican-bornchildren,thoughatafasterclip.Theadopteesandnativechildrenstartedcombiningwordsinsentenceswhentheirvocabularyreachedthesamesizes,furthersuggestingthatwhatmattersisnothowoldyouareorhowmatureyourbrainis,butthenumberofwordsyouknow.
Thisfinding--thathavingmorematurebrainsdidnothelptheadopteesavoidthetoddler-talkstage--suggeststhatbabiesspeakinbabytalknotbecausetheyhavebabybrains,butbecausetheyhaveonlyjuststartedlearningandneedtimetogainenoughvocabularytobeabletoexpandtheirconversations.Beforelong,theone-wordstagewillgivewaytothetwo-wordstageandsoon.
Learninghowtochatlikeanadultisagradualprocess.
Butthispotentialansweralsoraisesanevenolderandmoredifficultquestion.Adultimmigrants,wholearnasecondlanguagerarely,achievethesameproficiencyinaforeignlanguageastheaveragechildraisedasanativespeaker.Researchershavelongsuspectedthereisa"criticalperiod"forlanguagedevelopment,afterwhichitcannotproceedwithfullsuccesstofluency.Yetwestilldonotunderstandthiscriticalperiodorknowwhyitends.
Whatisthewriter'smainpurposeinParagraph2? A．Torejecttheviewthatadoptedchildrenneedtwolanguages.
B．Toarguethatcultureaffectsthewaychildrenlearnalanguage.
C．Togivereasonswhyadoptedchildrenwereusedinthestudy.
D．Tojustifyaparticularapproachtolanguagelearning.
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