‘When the doctors came they said she had died of heart disease - of joy that kills.’ Which one of…

‘When the doctors came they said she had died of heart disease - of joy that kills.’ Which one of the following explains why the short story, The Story of an Hour ends with the phrase ‘...of joy that kills’? (I) To convey a sense of irony. (II) To make the readers judge Mrs. Mallard. (III) To convey Mrs. Mallard’s sheer sense of shock. (IV) To explain Brently Mallard’s shock at the incident.