ReadthefollowingpassageandmarktheletterA,B, C,orD onyouranswersheettoindicatethecorrectanswertoeachofthe questions.

InScience,atheoryisareasonableexplanationofobservedeventsthatare related.Atheoryofteninvolvesanimaginarymodelthathelpsscientistspicture thewayanobservedeventcouldbeproduced.Agoodexampleofthisisfound inthekineticmoleculartheory,inwhichgasesarepicturedasbeingmadeup of many small particles that are in constantmotion.

Afteratheoryhasbeenpublicized,scientistsdesignexperimentstotestthe theory.Ifobservationsconfirmthescientists’predictions,thetheoryis supported.Ifobservationsdonotconfirmthepredictions,thescientistsmust searchfurther.Theremaybeafaultintheexperiment,orthetheorymayhave to be revised or rejected.

Scienceinvolvesimaginationandcreativethinkingaswellascollecting informationandperformingexperiments.Factsbythemselvesarcnotscience. AsthemathematicianJulesHenriPoincaresaid:"Scienceisbuiltwithfacts justasahouseisbuiltwithbricks,butacollectionoffactscannotbecalled science any more thana pile ofbricks can be called a house."

Mostscientistsstartaninvestigationbyfindingoutwhatotherscientistshave learnedaboutaparticularproblem.Afterknownfactshavebeengathered,the

scientistcomestothepartoftheinvestigationthatrequiresconsiderable imagination.Possiblesolutionstotheproblemareformulated.Thesepossible solutionsarecalledhypotheses.Inaway,anyhypothesisisaleapintothe unknown.Itextendsthescientist'sthinkingbeyondtheknownfacts.The scientistplansexperiments,performscalculations,andmakesobservationsto testhypotheses.Forwithouthypotheses,furtherinvestigationlackspurpose anddirection.Whenhypothesesareconfirmed,theyareincorporatedinto theories.

The word “threat” in paragraph 2 could best be replaced by ___________.