Letter is related to Unit. Unit is related to Bullet. Bullet is related to People. People is the opposite of Animal. 
Bus is part of Fleet. Fleet is part of Airline. Airline is related to Seat. 
Letter <--RelatedTo--> Unit <--RelatedTo--> Bullet <--RelatedTo--> People <--Antonym--> Animal 
Bus --PartOf--> Fleet --PartOf--> Airline <--RelatedTo--> Seat 