Answer:
C and C
Explanation:
Animals' genetic codes are what makes them what they are. Any slight difference in the genetic code makes them different from any other animal.
Natural selection is the survival of the fittest. If the environment changes rapidly, lets say, for example, it suddenly gets super cold where the swallows are. Then, the chicks that are more cold resistant (because of their different genetic codes) will be more likely to survive than the chicks that are not very cold resistant.
Hope this helped! :D