In the Gulf of Mexico Dead Zone, Oxygen Levels in Lower

Multiple Choice

In the Gulf of Mexico dead zone, oxygen levels in lower water levels are low because ____.

A) the runoff into the dead zone is already extremely low in oxygen and mixes with the water already present in the Gulf of Mexico
B) phytoplankton consume vast quantities of oxygen without producing any oxygen
C) pollutants in runoff, especially fertilizers, lead to phytoplankton growth and the dead phytoplankton are consumed by bacteria that use up oxygen
D) the runoff into the dead zone is contaminated with high densities of bacteria that consume oxygen
E) of the Great Lakes