oxygen sunlight nitrogen carbon dioxide
I gave my students this multiple choice question: When nitrogen-rich runoff from farms enters a nearby farm, it causes abundant growth of algal mat on the pond surface. As a result, underwater plants in the pond begin to die. Identify the limiting factor that is most responsible for causing the underwater plants to die.

Only one answer is correct among these 4 choices:
- oxygen
- sunlight
- nitrogen
- carbon dioxide

Could you tell me which one is correct?

sunlight