According to Darwin's theory of natural selection, the reasons for this phenomenon are as follows: (1) there are dark and light color variations in the offspring of birch, and its natural enemy birds play a selective role in the body color of birch; When the body color is consistent with the environmental color, it is not easy to be found by natural enemies, but it can survive and produce offspring. When the body color is inconsistent with the color of the environment, it is easy to be found and eaten by natural enemies, and the opportunity to produce offspring is lost, so there are fewer and fewer individuals with different colors from the environment.
So choose: B.