Fact 1: Migration is a type of movement that changes population size.

Fact 2: Birds migrate to escape poor weather and food supply.

Given the two facts above, answer the question "Escaping poor weather and food supply changes what?" with the following options: 
- population age
 - the environment
 - population education
 - general preferences
 - important habitats
 - population size
 - movement
 - an organism's body


 

      population size   