Fifty years ago, bed bugs were nearly eliminated in the united states by using of pesticides like ddt. today bed bugs are back in beds and theaters. ddt is no longer used due to environmental issues. pyrethrums are currently the top choice for bed bug infestations. pyrethrums are especially useful to us because they generally have a stronger effect on bugs than on humans and animals. some bed bugs had mutations in their genetic code which allowed them to survive the chemical pesticide and they produced chemically resistant offspring. how can this be explained?

Natural selection

Explanation:

What happened in the case of bed bugs and pesticides is natural selection at play. According to the natural selection theory propounded by Darwin, the environment selects for genes that able to withstand changes and organisms with such genes are able to survive, mate and pass on the selected gene to their offspring. Organisms without the selected gene gradually fades off from the population since they are not able to withstand the changes to the environment. Hence, after some considerable time, the desirable trait becomes dominant in the population.  

When DDT was in use, it was effective against bed bugs mutation in the genome of a few of the bed bug population conferred resistance against DDT on them. The environment selects this resistance trait because it is desirable. The resistant strains therefore, survive, mate and pass the gene to their offspring. After a while, bed bugs without the DDT-resistant gene die off the population while those with the resistant gene take over.

This might eventually happen to Pyrethrums too in the nearest future if natural selection law is to be considered.

b would be the right answer