Final answer:
In Stage 3 of the Demographic Transition Model, birth rates decline due to the shift from a rural, agricultural society to an urban, industrialized one, where children no longer provide economic benefits, and social security systems replace the need for children to support their aging parents.
Step-by-step explanation:
The declining Crude Birth Rate (CBR) during Stage 3 of the Demographic Transition Model (DTM) can be explained by the major societal changes that occur as a country transitions from a rural, agricultural society to an urban, industrialized one. At this stage, birth rates start to plummet, marking a shift in the way families perceive the value of having children. In the traditional rural society, children are often seen as economic assets who can work and contribute to the family's income. However, with urbanization and increased access to education, children become economically burdensome. Moreover, as social security systems improve, the necessity for children to provide old-age support diminishes. All these factors lead to families deciding to have fewer children, thus, the birth rate begins to fall significantly in Stage 3.