Grade is related to School. School is related to Subject. 
Fire is related to Ash. you are likely to find Ash in/at/on Air. Air is related to Necessary. Necessary is similar to Required. 
Grade <--RelatedTo--> School <--RelatedTo--> Subject 
Fire <--RelatedTo--> Ash --AtLocation--> Air <--RelatedTo--> Necessary <--SimilarTo--> Required 