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, escaping poor weather and food supply changes what?

Options are:
(1). population age
(2). the environment
(3). population education
(4). general preferences
(5). important habitats
(6). population size
(7). movement
(8). an organism's body
The answer is:
(6).