According to demographers, zero population growth occurs during the process of demographic transition when

the crude birth and death rates are simultaneously low.
the crude birth and death rates begin to increase rapidly.
the crude death rate is higher than the crude birthrate.
the total fertility rate falls below the crude birthrate.
the total fertility rate rises above the crude death rate.

The crude birth and death rates are simultaneously low.

Explanation:

A demographic change model incorporates 4 phases: low development, high development, diminishing development, and low development with ZPG. Diminishing development possibly happens when birth rates start to rapidly decay, and passing's rate decrease too, in which normal rates start to direct.

At long last, a low development rate as the fourth stage possibly happens when very low birth and demise rates make no long haul characteristic increment and conceivably a reduction.

Low development organize frequently are went with ZPGs (zero populace development), which happen when CBR and CDR are almost equivalent and NIR approaches 0.

separation of church and state? not sure what you mean.