Which of These Is an Example of Natural Selection Acting

Multiple Choice

Which of these is an example of natural selection acting on genetic variation resulting in evolutionary adaptation?

A) As global warming has continued, many species that were found near the tropics have migrated either northward or southward toward the poles.
B) During years of drought, tree rings are narrower.
C) As the African environment became drier and the grasslands more prevalent, arched feet and non-opposable toes, both more effective for walking upright, became more common in hominins.
D) Insect diversity in temperate broadleaf forests varies from year to year based on the amount of precipitation.