Each Summer, a Large "Dead Zone" Devoid of Fish and Other

Each summer, a large "dead zone" devoid of fish and other animals develops where the Mississippi River flows into the Gulf of Mexico. What causes this?

A) There are high levels of mercury in the river water.
B) Global warming is making the river water too hot for the fish to tolerate.
C) Acid rain makes the gulf's water too acidic to support fish.
D) High levels of pesticides are used in the Mississippi River basin.
E) Large quantities of fertilizer run off from Midwestern farm fields into the Mississippi River.