Concept:Rapid population growth happens when the number of births stays high while the number of deaths falls sharply.Explanation:A high death rate and a low birth rate would make the population shrink or grow very slowly.Using family planning methods reduces the birth rate, so it cannot cause rapid growth.The absence of natural hazards only prevents sudden large-scale deaths; it does not directly increase births or growth rate.Improved medical care lowers death rates by saving more babies and mothers during childbirth.When fewer people die and more babies survive, the population increases quickly.Answer:D. improved medical care