阅读下面材料，在空白处填入适当的内容(1个单词)或括号内单词的正确形式。

        Arewetooclean?

  Insomepartsoftheworlditseemsthat21____(keep)thingstoocleanmaycauseahealthproblem.Parentsoftenthinkthatchildrenarealwaysgettingdirty.Butthe22____(true)isthatincountriesliketheUKparentscleantheirhomessocarefully23____childrenhardlyevercomeintocontactwithdirtandthereforegerms(细菌).Theresultisthatonepersonintentoday24____(suffer)fromsomekindofallergy(过敏反应)一threetimesmorethanjusttenyearsago.

  Parentswhothinkbacktohowtheyusedtoplayasachildwillseehowmuchthings25____(change).Childrentodayspendalotoftimeintheirrooms.Twentyyearsagoourparentsthoughtitwasbetterforkids26____(play)outsidesowewouldspend27____(much)timeintheparkthanathome.Wealwaysfellover28____wejustbrushed29____(our)cleanandthenkeptonplaying.

  Nowresearchseemstosuggestthatthisis30____goodthing.Earlycontactwithgermshelpstoprotectourbodiesagainstallergieslaterinlife.