question_answer

A city was intensively sprayed with DDT in 1953 in an effort to control houseflies. The number of flies was immediately greatly reduced. Each year thereafter the city was again sprayed with DDT but the flies gradually increased in number until ten years later they were almost as abundant as they were when the control program began. Which one of the following is the most likely explanation?

A)  Flies from other areas moved in and replaced those killed by DDT

B)  The few flies that were affected by DDT got survived and developed resistance to DDT which they passed on to their descendants

C)  The DDT caused new mutations to occur in the surviving flies and this resulted in resistance to DDT

D)  The DDT killed susceptible flies but the few that were naturally resistant lived and reproduced and their offspring repopulated the area