Please choose the correct answer to the 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.

Answer1: oxygen Answer2: sunlight Answer3: nitrogen Answer4: carbon dioxide

Answer format: answer1/answer2/answer3/answer4
Answer: answer2