One of the following statements was given in the film as the best way to understand genetic differences in human populations. Choose the correct statement that also offers the correct explanation.

a. Populations differ by distance…If we were to walk from the tropics to Norway, what we would see is a continuous change in skin tone. And at no point along that trip would we be able to say this is the place in which we go from the dark race to the light race.
b. For race to be more than skin deep, one has to have concordance. In other words, skin color needs to reflect things that are deeper in the body under the skin. But most of human variation is non-concordant. Skin color…is not correlated with height or weight and definitely not correlated with more complex traits like intelligence or athletic performance.
c. It’s one of the misconceptions that sickle cell disease is an African American or an African disease. Sickle cell trait is not uncommon in people from the Mediterranean region…Where malaria was common the sickling gene was selected—in Arabia, South Asia, Central and Western, but not Southern Africa.
d. If one were to walk across the globe, you would find that in colder climates, people are lighter in skin color due to less melanin production. In warmer climates, you would find people are darker in skin color due to more melanin production. This is a survival mechanism.