Answer:
Choice D., elevated phosphorous and nitrogen in the water
Step-by-step explanation:
The dead zone is caused by the enrichment of nutrients from the river, especially nitrogen and phosphorus. Most of the nitrogen supply comes from the main agricultural states.
Nitrates and phosphorus used in fertilizers have a key effect once they reach the river. They stimulate an explosive growth of algae, which when they die fall to the bottom of the sea and decompose.
The bacteria that break down the algae consume oxygen, in a process that drastically reduces the level of gas available for marine life.