A number of mosquito populations today are resistant to insecticides that were once quite effective. Biologists think that insecticide resistance evolved in mosquitoes because __________.

• A. individual mosquitoes built up an immunity to an insecticide after being exposed to it
• B. mosquitoes needed to develop insecticide resistance to survive after the insecticide was used
• C. mosquitoes attempted to adapt to their environment
• D. a new allele developed in response to the insecticide that provided future generations the benefit of resistance
• E. a few mosquitoes were probably resistant to the insecticide before it was ever used, and these individuals were more likely to survive and reproduce in the presence of insecticide