Debt is related to Tax. Tax is related to Fee. Fee is related to Cost. Cost is the opposite of Revenue. 
Child is related to Adopt. Adopt is the opposite of Biological. 
Debt <--RelatedTo--> Tax <--RelatedTo--> Fee <--RelatedTo--> Cost <--Antonym--> Revenue 
Child <--RelatedTo--> Adopt <--Antonym--> Biological 