If a country has zero total growth but both births and deaths are very high, why is this different from zero growth with low births and low deaths?

Correct answer: Their demographic dynamics and age structures may differ

Zero total growth means that the final population is unchanged overall, but it does not reveal how many births and deaths occurred on the way. A country with high births and high deaths may have rapid population turnover, while one with low births and low deaths may have much less movement between age groups. Their age structures, health conditions, life expectancy, and future growth potential can therefore differ.

Thus choice A is correct: their demographic dynamics and age structures may differ. For example, equal high births and deaths can support a relatively young population and greater instability, whereas low births and deaths may be associated with an older, more stable population. Equal totals do not mean equal processes. Births and deaths remain important even when their net effect is zero.