The correct answer is - The population decreases.
A population that doesn't have people moving out of the country, nor people moving inside its borders, as well as high mortality rate, and low birth rate, is a population that will be decreasing rapidly, and that can lead in the end to its total disappearance.
At first the population will start to age, as there will be more middle aged and old people, and much less youngsters. As the time will pass, there will be less and less people for reproduction, and on top of that the ones that do, have one to two children at maximum. The old population will die out, and the numbers will start to significantly drop of the population, as the old and middle aged population are dominating the demographic pyramid.
