Were a Country to Have,over the Long Run,a Constant Subreplacement

Were a country to have,over the long run,a constant subreplacement level of fertility of 1,how many years would it take for the population of that country to halve?

A) slightly less than 20 years
B) slightly less than 30 years
C) slightly less than 50 years
D) slightly less than 70 years