जनसांख्यिकीय संक्रमण सिद्धांत (Demographic Transition Theory) क्या व्याख्या करता है?

Crude birth rate is defined as the number of live births per 1,000 population in:

Many developing countries experience continued population growth despite a decline in fertility rates due to which of the following factors? (a) Increase in life expectancy (b) Increase in infant mortality rate (c) Falling death rate (d) Better living conditions