Treat is a synonym of Process. Process is related to Division. Division is related to Equation. 
Flash is related to Photo. you are likely to find Sky in/at/on Photo. you are likely to find Pollution in/at/on Sky. 
Treat <--Synonym--> Process <--RelatedTo--> Division <--RelatedTo--> Equation 
Flash <--RelatedTo--> Photo <--AtLocation-- Sky <--AtLocation-- Pollution 