Aside from low birthrates and high deathrates, what can cause a country's population to decline?

Which of the following best summarizes the point of agreement?

If scientists only detect p waves in a certain layer of earths interior what does that tell us?

1. crust 1 the metallic layer at the earth's center 2. inner core 3 the firm layer between the crust and the asthenosphere 3. lower mantle 2 the layer surrounding the inner core 4. asthenosphere 4 the semi-solid layer between the top layer and the upper mantle 5. upper mantle 5 the layer between the asthenosphere and lower mantle 6. top layer 6 the earth's outermost layer 7. outer core 7 the hottest part of the mantle