A number of mosquito populations today are resistant to specific insecticides, even though those insecticides were highly effective when they were first introduced. Biologists believe that insecticide resistance evolved in mosquitoes because A. a few mosquitoes were probably resistant to the insecticide before it was ever used. These survived and passed that trait on to their offspring. B. mosquitoes needed to develop insecticide resistance to survive after the insecticide was used. C. the presence of the insecticide caused some mosquitoes to develop resistance; they survived and passed the trait to their offspring. D. current mosquito populations are phenotypically variable and exhibit differential fitness in different environments. E. individual mosquitoes built up an immunity to an insecticide after being exposed to it.