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. 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.

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

Explanation:

Natural selection is evolutionary mechanism which favours traits of an organism that help him survive and reproduce. In the example, above,  naturally resistant DDT flies that survived after the use of pesticide had the favuoruable trait (resistance) by natural selection.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.