A Number of Different Genes Have Been Identified as Affecting

A number of different genes have been identified as affecting skin color and there are probably more as yet unidentified. Each of these genes likely has several regulatory protein factors affecting their expression, and those proteins are coded for by yet more genes. Thus there are many genes, some of which have multiple alleles, involved in determining skin color. What property of skin color does this multigene foundation not explain?

A) Some people tan easily and some get sunburned rather than becoming tan.
B) Some people have very dark skin, some have very light skin, and there is nearly continuous variation in between.
C) There is variation in the amount of vitamin D humans need.
D) Skin color can be acted on by natural selection.