Final answer:
The Demographic Transition Theory indicates that significant demographic change is primarily caused by changes in birth and death rates as a country develops from pre-industrial to industrialized.
Step-by-step explanation:
According to the Demographic Transition Theory, significant demographic change is caused by B) Changes in birth and death rates. The theory posits that as a country or region develops from a pre-industrial to an industrialized economic system, there is a transition from higher birth and death rates to lower birth and death rates. This process usually begins with a decline in death rates due to improvements in medical technology and sanitation, which results in a net population gain. Eventually, birth rates also begin to decline, leading to a convergence of birth rates and death rates at a lower level. Although factors like migration, urbanization, economic development, and government policies may influence demographic change, they are not the primary focus of the Demographic Transition model.