Acity was intensively sprayed with ddt to control houseflies. the number of houseflies was immediately greatly reduced. each year thereafter, the city was sprayed again, but the flies gradually increased in numbers until 10 years later, when they were almost as abundant as they were when the control program began. what most likely explains this situation?

The correct answer is "The houseflies developed DDT resistance".

Explanation:

(DDT) is a chemical compound originally developed as insecticide, which was later banned in most countries for its environmental impact. In the scenario stated in the question the houseflies were exposed to DDT intermittently, resulting in an initial reduce of the houseflies population and in a gradually increased until restoring its population 10 years later. This is explained by the houseflies developed DDT resistance. Initially all the houseflies that were susceptible to DDT died, but the few that were naturally resistant survived and had offspring. This process occurs in nature and DDT resistance has been reported in different species of mosquitoes.