Roof is part of Bus. you are likely to find Water in/at/on Roof. Water is a different concept from Grass. Grass is related to Sheep. 
Opportunity is related to Choose. Choose entails/necessarily involves Compare. 
Bus <--PartOf-- Roof <--AtLocation-- Water <--DistinctFrom--> Grass <--RelatedTo--> Sheep 
Opportunity <--RelatedTo--> Choose --Entails--> Compare 