Question

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 ten years later they were almost as abundant as they were when the control program began. Which of the following most likely explains this?
A. Flies from other areas moved in and replaced the ones killed by DDT.
B. The few flies that were affected by DDT but survived developed antibodies to DDT which they passed on to their descendants.
C. The DDT caused new mutations to occur in the surviving flies resulting in resistance to DDT.
D. The DDT killed susceptible flies. The few that were already naturally resistant survived and passed this resistance on to their offspring.
E. DDT is only affective in dry years. Eventually another very dry year will occur and DDT will again wipe out the flies in the city.

Answer 1

Answer:

Option D, The DDT killed susceptible flies. The few that were already naturally resistant survived and passed this resistance on to their offspring.

Explanation:

When DDT was sprayed for the first time, some of the house flies which were not capable enough to withstand the adverse effects of DDT , eventually died. And the butterflies that survived through this,   were having a natural resistance against DDT. Now, with time these DDT resistant houseflies mated and produced offspring that had genetic trait of resisting DDT by birth. This is the reason,  why houseflies after ten years were as abundant as  ten years back when the control program was started.

Option D is correct