Oxygen is related to Component. Component is related to Speaker. Speaker is related to Part. 
Difficult is related to Heavy. Heavy is related to Pressure. Pressure is related to Weight. Weight is a type of Importance. 
Oxygen <--RelatedTo--> Component <--RelatedTo--> Speaker <--RelatedTo--> Part 
Difficult <--RelatedTo--> Heavy <--RelatedTo--> Pressure <--RelatedTo--> Weight --IsA--> Importance 