The correct answer is c. Genetic Variation. Bacteria are very numerous, ad random mutation of bacterial DNA generates a wide variety of change, including resistance.
Who is Dede and what are her symptoms. We need more context
If you talk about the process of fission in the nuclear field then the most used is U-235 because is the easiest to use. Have in mind that the majority of uses of uranium is U-238 but the easiest to be used is U-235
