完形填空
题干：AreweatthebeginningofanotherAgeofExploration?Perhapsevenmoreimportantly,areweatthebeginningof（1）AgeofColonization?Asthepopulationoftheworldincreasestowardsthepoint（2）theearthcannolongersupportallthepeople（3）onit,thesecondquestionbecomesurgent.Willwediscoveranewworld,（4）Columbusdid,onwhichhumanlifewillbepossible?Atthispointinthespaceage,noonecanreallyanswerthesequestions.Wecansay,（5）,thatwewillnotseetomorrowthekindofspacetravelthat（6）fictionandthemovieshaveshown.Itwillbealongtimebeforewehaveflightsthatrun（7）tohumancoloniesonthemoonorononeoftheplanets.Wearenotevengoingtobeableto（8）immediateadvantagesofthemineralsthatwemayfindontheplanets（9）ourownsolarsystem.Greatproblemsmustbesolved（10）wecouldsendcolonistsoutintospace.Thedistancesthatmustbe（11）andthelengthoftimeittakestodothatcanhardlybe（12）.Therearealsodangersthatwestilldonotreallyunderstandfromradiation,forexample,orfrompiecesofmatters（13）inspace,orfromcontaminationfromformsoflifethatmight（14）there.Thereisalsoneedforhumanstotaketheirownenvironmentintospace（15）them.Sofarno"island"hasbeendiscoveredinspaceonwhichpeoplecanexistwithoutsystemsthat（16）life,andthesesystemsmustaccompanyanyfuturespace（17）.Finally,onthemostpractical（18）,therehastobeenormousexpense（19）inspaceexploration.TheU.S.andRussiangovernmenthavealreadyspentbillionsofdollarsforprojects（20）whichtheycanreceiveareturnonlyinknowledgeandnotinmoney.
题目：（13）
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