The Mighty Mississippi River Flows into the Gulf of Mexico

The mighty Mississippi River flows into the Gulf of Mexico. The zone where the Mississippi River enters in the Gulf is known as the "dead zone". Here, few (if any) animals are able to live. Why is this?

A) The area has an overload of nutrients from the river, creating an algae bloom. When the algae die, their breakdown causes oxygen depletion.
B) The area has an overload of pesticides from agriculture. The pesticides kill everything in this zone.
C) The discharge of the river into the Gulf is so strong that animals are unable to leave there.
D) All of these.