Answer: B. a neighborhood where lawn chemicals are widely used
Why: Blue-green algae blooms in drinking water reservoirs are often associated with nutrient enrichment, particularly an excess of phosphorus and nitrogen. Lawn chemicals, such as fertilizers and pesticides, commonly contain these nutrients. Therefore, investigating a neighborhood where lawn chemicals are widely used is a reasonable first step, as it may reveal a potential source of nutrient runoff into the reservoir, contributing to the algae bloom.