Why do earthquakes occur only above the brittle/plastic transition depth?

What is one reason why areas that are just beginning to industrialize typically have a higher rate of population growth than countries that are already industrialized? a. most industrialized countries have laws regulating how many children women can have. b. the average lifespan in developing countries is much higher than in industrialized countries. c. developing countries need labor to fill their rapidly expanding industries. d. developing countries are just beginning to experience improved food production and health services.