Suppose that two mosquito populations are being studied. The populations differ in their choice of the water in which they choose to lay their eggs. One population is urban, and avoids laying eggs in water treated with insecticides which are heavily used in urban areas. What difference between populations would provide the best evidence that this preference is due to proximate causes?

• A difference in the mosquito’s ability to sense insecticide levels
• A genetic difference between the two populations
• A difference in preference for insecticide-free water
• A difference in reproductive behavior