Final answer:
The prevalent theory regarding the evolution of giraffe's long necks is that it is a result of the process of natural selection, where giraffes with longer necks had an advantage in accessing food on tall trees. Therefore, the hypothesis that animals constantly stretching their necks will grow longer necks (Scientist A's hypothesis) is not supported by the observed evidence and has been discarded.
Step-by-step explanation:
Scientist B conducted experiments and observed that stretching muscles does NOT cause bones to grow longer nor change the DNA of animals so that longer muscles would be passed on to the next generation. Scientist B, therefore, discarded Scientist A's suggested answer as to why all the animals living in the area had long necks. Scientist B suggested instead that originally many different types of animals including giraffes had lived in the region but only the giraffes could survive when the only food was high in the trees, and so all the other species had left the area.
The prevalent theory regarding the evolution of giraffe's long necks is that it is a result of the process of natural selection. Giraffes with longer necks had an advantage because they could reach leaves on tall trees that other giraffes couldn't access. As a result, long-necked giraffes were more likely to survive, reproduce, and pass on the long-neck trait to their offspring. Over time, this led to the entire giraffe population having long necks.
Therefore, the hypothesis that animals constantly stretching their necks will grow longer necks (Scientist A's hypothesis) is not supported by the observed evidence and has been discarded in favor of the theory of natural selection.