ReadthefollowingpassageandmarktheletterA,B, C,orD onyouranswersheettoindicatethecorrectanswertoeachofthe questions.

Earthisrichlysuppliedwithdifferenttypesoflivingorganismswhichco-existintheirenvironments,formingcomplex,interrelatedcommunities.Living organismsdependononeanotherfornutrients,shelter,andotherbenefits.The extinctionofonespeciescansetoffachainreactionthataffectsmanyother species,particularlyifthelossoccursnearthebottomofthefoodchain.For example,theextinctionofaparticularinsectorplantmightseem inconsequential.However,theremaybefishorsmallanimalsthatdependon thatresourceforfoodstuffs.Thelosscanthreatenthesurvivalofthesecreatures andlargerpredatorsthatpreyuponthem.Extinctioncanhavearippleeffect that spreads throughout nature.

In additiontoitsbiological consequences,extinctionposes amoral dilemma forhumans,theonlyspeciescapableofsavingtheothers.Thepresenceof humansontheplanethasaffectedallotherlifeforms,particularlyplantsand animals.Human lifestyles haveproventobe incompatiblewiththesurvival of someotherspecies.Purposefuleffortshavebeenmadetoeliminateanimals thatpreyonpeople,livestock,crops,orposeanythreattohumanlivelihoods. Somewildanimals havebeendecimatedbyhuman desireformeat,hides,fur, orotherbodypartswithcommercialvalue.Likewise,demandforland,water, andothernaturalresourceshasleftmanywildplantsandanimalswithlittleto nosuitablehabitat.Humanshavealsoaffectednaturebyintroducingnon-native speciestolocalareasandproducingpollutantshavinganegativeimpactonthe environment.Thecombinationofthesehuman-relatedeffectsandnatural obstaclessuch asdisease orlow birthrates hasproventobetoo muchforsome species to overcome. They have no chance of survival withouthuman help.

Asaresult,societieshavedifficultchoicestomakeabouttheamountofeffort andmoneytheyarewillingtospendtosaveimperiledspecies.Willpeople acceptlimitsontheirpropertyrights,recreationalactivities,andmeansof livelihood tosaveaplantorananimal?Shouldsavingsuchpopularspeciesas whalesanddolphinstakepriorityoversavingobscure,annoying,orfearful species?Is itthe responsibilityof humanstosaveeverykind of lifeformfrom disappearing,orisextinctionaninevitablepartofnature,inwhichthestrong surviveandtheweakperish?Thesearesomedifficultquestionsthatpeople face as they ponder thefate of other species livingon this planet.

The word "their" in paragraph 1 refers to ______.