Skill is related to Knowledge. Knowledge is a synonym of Light. Light is the opposite of Strong. Strong is a synonym of Firm. 
Abstract is related to End. End is related to Beginning. Beginning is a synonym of Element. Element is a synonym of Member. 
Skill <--RelatedTo--> Knowledge <--Synonym--> Light <--Antonym--> Strong <--Synonym--> Firm 
Abstract <--RelatedTo--> End <--RelatedTo--> Beginning <--Synonym--> Element <--Synonym--> Member 