When they were first sold, certain insecticides were highly effective in killing mosquitoes. today, dozens of mosquito generations later, a much smaller proportion of these insects die when sprayed with the same chemicals. fewer insects are killed today because

a. mosquitoes are deliberately adapting themselves to this man-made change in the environmentb. mosquitoes that survive spraying develop an immunity to the insecticidec. many mosquitoes today are descendants of mosquitoes with insecticide-resistant characteristicsd. the original spraying has caused a permanent mutation, giving the insects genetic resistance to the spray