Disability is related to Blind. Blind is related to Lack. Lack is related to Shortage. 
Must is related to Required. Required is a synonym of Due. Due is a synonym of Just. Just is the opposite of Unfair. 
Disability <--RelatedTo--> Blind <--RelatedTo--> Lack <--RelatedTo--> Shortage 
Must <--RelatedTo--> Required <--Synonym--> Due <--Synonym--> Just <--Antonym--> Unfair 