In the Context of Epidemiology, What Does It Mean to Describe

In the context of epidemiology, what does it mean to describe the world as "smaller"?

A) The size of Earth has been shrinking since it formed 4.56 billion years ago.
B) The size of the world's population is larger than Earth can sustain.
C) Humans can travel from continent to continent within hours or days.
D) The continents are shifting in a way to make the land on Earth smaller.