题目内容：
Questionsarebasedonthefollowingpassage:
(51)It'sadaytocelebrate,atimetoeducate,andawaytomakeadifference．EarthDaywasfirstobservedonApril22,1970.ThetraditionbeganintheUnitedStatesandisnowaninternationalfocalpointforpeoplewhoareconcernedabouttheenvironment.
ImagineatimebeforetheCleanAirAct,theCleanWaterAct,theEndangeredSpeciesAct,andeventheEnvironmentalProtectionAgency．Hottopicsofthetimewereindustrialpollution,nuclearaccidents,oildependency,pesticides,andavarietyofsocialissues.However,policymakerswerenotyetactingonmanyimportantenvironmentalproblems.
EarthDaygrewoutofgrassrootsactionandinstitutionalSupport．StudentleaderDenisHayesandWisconsinSenatorGaylordNelsonweretwokeyorganizersforthefirstevent．Over20millionconcernedcitizensralliedforcleanairandwater.Collegestudentswereamajorforceinthecampaign,althoughparticipantsofallageswereinvolved．Still,wecannotreallycreditahandfulofindividualsforthesuccessofEarthDay．Afterall,amajormessageofEarthDayisthateverysinglepersonhasanimportantroleintheenvironment.
(52)ThefirstEarthDaywasverysuccessful,inspiringthepassageofcongressionalprotectionforair,water,andendangeredspecies．MomentumfromthedemonstrationalsofueledtheformationoftheEnvironmentalProtectionAgency.
Thirtyyearslater,thefocushaschangedbuttheurgencyremainsthesame．Duetoincreasingamountsofscientificevidencethatfossilfuelsarechangingtheglobalclimate,thethemeforEarthDay2000isCleanEnergy．ThefourmajorgoalsofEarthDay2000arecleancam,cleanpower,cleanair,andcleaninvestments.
April22nd,2000isthe__________years'anniversaryoftheEarthDay. A．30th
B．20th
C．10th
D．100th
参考答案：
答案解析：