Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: 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.
sunlight
Lack of sunlight.