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: A drinking water reservoir for a small town develops a blue-green algae bloom. The town hires a company to eliminate the problem. Which of the following should the company investigate first as a likely source of the problem?
a neighborhood where lawn chemicals are used
Nutrient levels in the water.