What Is One Explanation for Why People Are Getting Taller

What is one explanation for why people are getting taller in many regions of the world?

A) People in those regions have better nutrition and lower rates of infectious disease than they did in the past.
B) People in those regions have better medical care than people in other parts of the world.
C) People across the world are getting taller due to gene flow from taller populations.
D) People are getting taller because tall people are more likely to have children, and the genes are becoming more frequent.