**Core Concept**
Demographic transition is a model that describes the changes in population growth and structure as a country develops from a pre-industrial, agrarian society to an industrialized, post-industrial society. The model consists of five stages, each characterized by distinct demographic features, including birth and death rates.

**Why the Correct Answer is Right**
In stage 3 of the demographic transition, the crude birth rate (CBR) begins to decline, while the crude death rate (CDR) has already started to decrease. This is because improvements in sanitation, hygiene, and healthcare lead to a decrease in mortality rates, particularly among children and the elderly. However, fertility rates remain high due to social and economic factors, resulting in a continued increase in population growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because in stage 1 of the demographic transition, both CBR and CDR are high, and population growth is rapid.
**Option B:** This option is incorrect because in stage 2 of the demographic transition, CBR is high, but CDR is also high, and population growth is still rapid.
**Option C:** This option is incorrect because in stage 4 of the demographic transition, CBR has already started to decrease, and CDR continues to decrease.

**Clinical Pearl / High-Yield Fact**
The demographic transition model is essential in understanding population growth and structure, which can inform public health policy and resource allocation. It is crucial to recognize the distinct characteristics of each stage to make informed decisions about healthcare and social services.

**Correct Answer:** C.