What Factors Led to Confusion and Concern Over the Exploding

What factors led to confusion and concern over the exploding population growth in the mid-1900s?

A) Fertility rates declined,but mortality rates did not.It was not known then that mortality rates would decline shortly thereafter.
B) A massive influx of immigrants occurred.It was not known then that immigration rates would quickly decline.
C) Mortality rates declined,but fertility rates did not.It was not known then that fertility rates would fluctuate for a few more decades.
D) Mortality rates declined,but fertility rates did not.It was not known then that fertility rates would decline shortly thereafter.