When they were first sold, aerosol insecticides were highly effective in killing flies and mosquitoes. Now, several decades later, a much smaller proportion of these insects from a given species die when sprayed. Which of the following is a reason fewer insects die when they are sprayed?

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