B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### 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.

### Choices:
A. oxygen
B. sunlight
C. nitrogen
D. carbon dioxide

### Answer:
