Which of the Following Best Explains Why Middle Adulthood Starts

Which of the following best explains why middle adulthood starts later and lasts longer now than in the early 1900s?

A) People today have fewer children than people in the early 1900s.
B) People today have a higher income than people in the early 1900s.
C) People today have less anxiety in their lives than people in the early 1900s.
D) People today have healthier lifestyles than people in the early 1900s.