Silk is related to Elegant. Elegant is related to Sophisticated. Sophisticated is the opposite of Plain. 
Superior is the opposite of Bad. Bad is related to Bitter. Bitter is the opposite of Optimistic. 
Silk <--RelatedTo--> Elegant <--RelatedTo--> Sophisticated <--Antonym--> Plain 
Superior <--Antonym--> Bad <--RelatedTo--> Bitter <--Antonym--> Optimistic 