Gas is related to Chemical. Cocaine is a type of Chemical. Cocaine is a synonym of Rock. 
Plain is a synonym of Apparent. Apparent is a synonym of Obvious. Obvious is related to Clear. Clear is related to Crystal. 
Gas <--RelatedTo--> Chemical <--IsA-- Cocaine <--Synonym--> Rock 
Plain <--Synonym--> Apparent <--Synonym--> Obvious <--RelatedTo--> Clear <--RelatedTo--> Crystal 