A city 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. Which explanation applies to this situation?The DDT killed most flies. The few that were already naturally resistant survived and passed this resistance on to their offspring.Flies from other areas moved in and replaced the ones killed by DDT.The DDT caused new mutations to occur in the surviving flies, resulting in resistance to DDT.The few flies that were affected by DDT but survived developed antibodies to DDT, which they passed on to their descendants.