Becauseyoucanarguethatifyourcross-priceelasticityispositiverelativetoanotherproduct,youthendonotpossessanyexclusivenesstotheproduct.
Inanotherword,beingamonopolist,youpossessmarketpower,thisismainlybecauseyouaresellingaexclusivegood,thatthereisnowhereelsetheconsumercangetitfrom.
Soifyouriseyourprice,someconsumermayleave,buttheotherswillstay.
However,ifitisthecasethatthepriceelasticityofdemandofyourproductispositivecomparedtoothergoods,thatis,ifyouriseyourpriceby1percent,thedemandofasecondgoodincreasesbyacertainpercentage,thenitmustbetruethatthereexistsotherfirmsorcompanythatsellsgoodswhichcouldpotentiallyreplaceyourproduct,thenyouarenolongeramonopolistsinceyoudontproduceanexclusivegood.
Thisiswhyyoucouldusecross-priceelasticityofdemandtodeterminewhetherafirmisamonopolist
BTW:dontusewhatijustsaidforyourexam,itsjusttoocasual,taketheideasandbuilduponit.ECON111isboring,imofftoACST211hopeididhelpyou.