Period is related to Month. Month is related to Length. 
Shallow is the opposite of Long. Long is related to Rat. Rat is a type of Hire. Hire is a synonym of Engagement. 
Period <--RelatedTo--> Month <--RelatedTo--> Length 
Shallow <--Antonym--> Long <--RelatedTo--> Rat --IsA--> Hire <--Synonym--> Engagement 