Explanation:

- High biological oxygen demand: Dead zones occur when oxygen levels drop, often due to decomposing organic matter, which increases oxygen demand.

- Excessive bloom of algae: Algal blooms are common before dead zones form. When algae die, they decompose, reducing oxygen.

- Excess nutrients like nitrates and phosphates: Runoff containing these nutrients can fuel excessive algae growth, starting the process that leads to dead zones.

- All of the above: These factors interplay, creating and exacerbating dead zones.