阅读短文，选出填入空格最合适的一项： In many rich countrie…

阅读短文，选出填入空格最合适的一项：

In many rich countries, the population is getting older as fewer babies are born and people live longer. Several of these countries already have more people over sixty-five than under fifteen. ___, many countries in Africa have very young populations, with half of their people under the age of twenty. These differences will shape the job markets of the coming decades.