Under what conditions might an oxygen sag curve occur? 

A)  during spring floods 
B)  when oxygen-demanding wastes are added to the water 
C)  upstream from a sewage treatment plant 
D)  in a fast-flowing river

Oxygen Sag Curve

A graphical representation showing the decrease of dissolved oxygen in a body of water downstream from a pollution source as microorganisms decompose organic matter.

Oxygen-demanding Wastes

Wastes that require oxygen for their decomposition, which can deplete oxygen levels in water bodies, affecting aquatic life.

Sewage Treatment

The process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater.